Typical features of acne include increased secretion of oily sebum by the skin, microcomedones, comedones, papules, nodules (large papules), pustules, and often results in scarring. The appearance of acne varies with skin color. It may result in psychological and social problems.

The severity of acne vulgaris (Gr. ἀκµή, "point" + L. vulgaris, "common") can be classified as mild, moderate, or severe as this helps to determine an appropriate treatment regimen. Mild acne is classically defined by the presence of clogged skin follicles (known as comedones) limited to the face with occasional inflammatory lesions. Moderate severity acne is said to occur when a higher number of inflammatory papules and pustules occur on the face compared to mild cases of acne and are found on the trunk of the body. Severe acne is said to occur when nodules (the painful 'bumps' lying under the skin) are the characteristic facial lesions and involvement of the trunk is extensive.

Large nodules were previously referred to as cysts, and the term "nodulocystic" has been used in the medical literature to describe severe cases of inflammatory acne. True cysts are in fact rare in those with acne and the term "severe nodular acne" is now the preferred terminology.

"Acne inversa" (L. invertō, "upside down") and "acne rosacea" (rosa, "rose-colored" + -āceus, "forming") are not true forms of acne and respectively refer to the skin conditions hidradenitis suppurativa (HS) and rosacea. Although HS shares certain common features with acne vulgaris, such as a tendency to clog skin follicles with skin cell debris, the condition otherwise lacks the defining features of acne and is therefore considered a distinct skin disorder.

Seborrhoeic dermatitis' symptoms appear gradually, and usually the first signs are flaky skin and scalp. Symptoms occur most commonly anywhere on the skin of the scalp, behind the ears, on the face, and in areas where the skin folds. Flakes may be yellow, white or grayish. Redness and flaking may also occur on the skin near the eyelashes, on the forehead, around the sides of the nose, on the chest, and on the upper back.

In more severe cases, yellowish to reddish scaly pimples appear along the hairline, behind the ears, in the ear canal, on the eyebrows, on the bridge of the nose, around the nose, on the chest, and on the upper back.

Commonly, patients experience mild redness, scaly skin lesions and in some cases hair loss. Other symptoms include patchy scaling or thick crusts on the scalp, red, greasy skin covered with flaky white or yellow scales, itching, soreness and yellow or white scales that may attach to the hair shaft.

Seborrhoeic dermatitis can occur in infants younger than three months and it causes a thick, oily, yellowish crust around the hairline and on the scalp. Itching is not common among infants. Frequently, a stubborn diaper rash accompanies the scalp rash. Usually, when it occurs in infants the condition resolves itself within days and with no treatment.

In adults, symptoms of seborrhoeic dermatitis may last from a few weeks, to years. Many patients experience alternating periods of inflammation. The condition is referred to a specialist when self-care has proven unsuccessful.

Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin.

A comedo is a clogged hair follicle (pore) in the skin. Keratin (skin debris) combines with oil to block the follicle. A comedo can be open (blackhead) or closed by skin (whitehead), and occur with or without acne. The word comedo comes from the Latin "comedere", meaning "to eat up", and was historically used to describe parasitic worms; in modern medical terminology, it is used to suggest the worm-like appearance of the expressed material.

The chronic inflammatory condition that usually includes both comedones and inflamed papules and pustules (pimples) is called acne. Infection causes inflammation and the development of pus. Whether a skin condition classifies as acne depends on the amount of comedones and infection. Comedones should not be confused with sebaceous filaments.

Comedo-type ductal carcinoma in situ (DCIS) is not related to the skin conditions discussed here. DCIS is a non-invasive form of breast cancer, but comedo-type DCIS may be more aggressive and so may be more likely to become invasive.

Seborrhoeic dermatitis, also known as seborrhea, is a long term skin disorder. Symptoms include red, scaly, greasy, itchy, and inflamed skin. Often the scalp, face, and chest are affected. It can result in social or self-esteem problems. In babies, when the scalp is primarily involved, it is called cradle cap. Dandruff is a milder form of the condition, without associated inflammation.

The cause is unclear but believed to involve a number of genetic and environmental factors. Risk factors include poor immune function, Parkinson disease, epilepsy, and Down syndrome. The condition may worsen with stress or during the winter. It is not a result of poor hygiene. Diagnosis is typically based on the symptoms.

The typical treatment is antifungal cream and anti-inflammatory agents. Specifically ketoconazole or ciclopirox are effective. It is unclear if other antifungals, such as miconazole, are equally effective as they have been poorly studied. Other options may include coal tar and phototherapy.

The condition is most common in those around the age of 50, during puberty, and among those less than three months old. In adults about 2% of people are affected. Males are more often affected than females. Up to 40% of babies may be affected to some degree.

Excoriated acne (also known as Picker's acne or Acne excoriée des jeunes filles) is a mild acne accompanied by extensive excoriations.

Oil production in the sebaceous glands increases during puberty, causing comedones and acne to be common in adolescents. Acne is also found premenstrually and in women with polycystic ovarian syndrome. Smoking may worsen acne.

Oxidation rather than poor hygiene or dirt causes blackheads to be black. Washing or scrubbing the skin too much could make it worse, by irritating the skin. Touching and picking at comedones might cause irritation and spread infection. It is not clear what effect shaving has on the development of comedones or acne.

Some, but not all, skin products might increase comedones by blocking pores, and greasy hair products (like pomades) can worsen acne. Skin products that claim to not clog pores may be labeled noncomedogenic or non-acnegenic. Make-up and skin products that are oil-free and water-based may be less likely to cause acne. It is not known whether dietary factors or sun exposure make comedones better, worse or have no effect.

A hair that does not emerge normally can also block the pore and cause a bulge or lead to infection (causing inflammation and pus).

Genes may play a role in the chances of developing acne. Comedones may be more common in some ethnic groups. People of recent African descent may experience more inflammation in comedones, more comedonal acne, and earlier onset of inflammation.

Acne fulminans begins as pain and inflammation in the joints. It eventually progresses into a swelling of the lymph nodes located at the base of the neck, causing inflexibility in the neck within weeks after the nodes swell. This swelling will eventually decrease, but this decrease will be accompanied by an increased inflammation and swelling of the joints, as well as a complete loss of appetite, though these symptoms are often ignored. After some time, the disease will cause an extreme loss of weight and atrophy of the muscles, leading to the decline of physical abilities.

Hyperpigmentation can be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris. People with darker skin tones are more prone to hyperpigmentation, especially with excess sun exposure.

Many forms of hyperpigmentation are caused by an excess production of melanin. Hyperpigmentation can be diffuse or focal, affecting such areas as the face and the back of the hands. Melanin is produced by melanocytes at the lower layer of the epidermis. Melanin is a class of pigment responsible for producing colour in the body in places such as the eyes, skin, and hair. As the body ages, melanocyte distribution becomes less diffuse and its regulation less controlled by the body. UV light stimulates melanocyte activity, and where concentration of the cells is greater, hyperpigmentation occurs. Another form of hyperpigmentation is post inflammatory hyperpigmentation. These are dark and discoloured spots that appear on the skin following acne that has healed.

Hyperpigmentation is associated with a number of diseases or conditions, including the following:

- Addison's disease and other sources of adrenal insufficiency, in which hormones that stimulate melanin synthesis, such as melanocyte-stimulating hormone (MSH), are frequently elevated.

- Cushing's disease or other excessive adrenocorticotropic hormone (ACTH) production, because MSH production is a byproduct of ACTH synthesis from proopiomelanocortin (POMC).

- Acanthosis nigricans—hyperpigmentation of intertriginous areas associated with insulin resistance.

- Melasma, also known as "chloasma"—patchy hyperpigmentation

- Acne scarring from post-inflammatary hyperpigmentation

- Linea nigra—a hyperpigmented line found on the abdomen during pregnancy.

- Peutz-Jeghers syndrome—an autosomal dominant disorder characterized by hyperpigmented macules on the lips and oral mucosa and gastrointestinal polyps.

- Exposure to certain chemicals such as salicylic acid, bleomycin, and cisplatin.

- Smoker's melanosis

- Coeliac disease

- Cronkite-Canada syndrome

- Porphyria

- Tinea fungal infections such as ringworm

- Haemochromatosis—a common but debilitating genetic disorder characterized by the chronic accumulation of iron in the body.

- Mercury poisoning—particularly cases of cutaneous exposure resulting from the topical application of mercurial ointments or skin-whitening creams.

- Aromatase deficiency

- Nelson's syndrome

- Grave's disease

- As a result of tinea cruris.

Hyperpigmentation can sometimes be induced by dermatological laser procedures.

Acneiform eruptions are a group of dermatoses including acne vulgaris, rosacea, folliculitis, and perioral dermatitis. Restated, acneiform eruptions are follicular eruptions characterized by papules and pustules resembling acne.

The hybrid term "acneiform", literally, refers to an appearance similar to acne.

The terminology used in this field can be complex, and occasionally contradictory. Some sources consider acne vulgaris part of the differential diagnosis for an acneiform eruption. Other sources classified acne vulgaris under acneiform eruption. MeSH explicitly excludes perioral dermatitis from the category of "acneiform eruptions", though it does group acneiform eruptions and perioral dermatitis together under "facial dermatoses".

The cause of HS remains unknown and experts disagree over proposed causes. The condition probably results from a combination of genetic and environmental factors.

Lesions occur in any body areas with hair follicles, although areas such as the axilla, groin, and perianal region are more commonly involved. This theory includes most of the following potential indicators:

- Postpubescent individuals are more likely to exhibit HS.

- Plugged apocrine (sweat) gland or hair follicle

- Excessive sweating

- Androgen dysfunction

- Genetic disorders that alter cell structure

- Patients with more advanced cases may find exercise intolerably painful, which may increase the rate of obesity among sufferers.

The historical understanding of the disease suggests dysfunctional apocrine glands or dysfunctional hair follicles, possibly triggered by a blocked gland, create inflammation, pain, and a swollen lesion.

Hidradenitis suppurativa is a chronic inflammatory skin condition, considered a member of the acne family of disorders. It is sometimes called acne inversa. The first signs of HS are small bumps on the skin that resemble pimples, cysts, boils, or folliculitis. As the disease progresses and abscesses reoccur, they become larger and more painful; eventually tunnels of scar tissue connect the lesions. These lesions may open up if they become too enlarged and drain bloodstained pus. One risk factor is age; HS usually first appears during the 20s and early 30s. The condition is much more common in women than in men but is usually more serious and debilitating in men. Other associated conditions include obesity, diabetes, metabolic syndrome, arthritis, acne, and other inflammatory disorders. Early diagnosis of this disease is very important to decrease the number of flares, pain, and discomfort.

Hidradenitis suppurativa (HS), also known as acne inversa, is a long term skin disease characterized by the occurrence of inflamed and swollen lumps. These are typically painful and break open releasing fluid or pus. The areas most commonly affected are the underarms, under the breasts, and groin. After healing scar tissue remains. Self consciousness or depression may result.

The exact cause is usually unclear but believed to involve a combination of genetic and environmental factors. About a third of people have an affected family member. Other risk factors include obesity and smoking. The condition is not infectious, related to hygiene, or to the use of deodorant. The underlying mechanism is believed to involve either a dysfunction of apocrine sweat glands or hair follicles. Diagnosis is based on the symptoms.

There is no known cure. Warm baths may be tried in those with mild disease. Cutting open the lesions to allow them to drain does not result in significant benefit. While antibiotics are commonly used evidence for their use is poor. Immunosuppressive medication may also be tried. In those with more severe disease laser therapy or surgery to remove the affected skin may be carried out. Rarely a skin lesion may develop into skin cancer.

It is estimated to affect between 1% and 4% of people. Females have the condition about three times more often than males. Onset is typically in young adulthood and may become less common after 50 years old. It was first described some time between 1833 and 1839 by Alfred Velpeau.

A pimple, zit or spot is a kind of comedo and one of the many results of excess oil getting trapped in the pores. Some of the varieties are pustules or papules. Pimples can be treated by various acne medications prescribed by a physician, or purchased at a pharmacy with a wide variety of treatments.

Halogen acne is caused by iodides, bromides and fluorides (halogens) that induce an acneiform eruption similar to that observed with steroids.

Oil acne is an occupational skin condition caused by exposure to oils used in industry.

Acne aestivalis (also known as Mallorca acne) is a special form of mono-morphic light eruption. It is a monomorphous eruption consisting of multiple, uniform, red, papular lesions, reported to occur after sun exposure.

Infantile acne is a cutaneous condition that presents at 3 to 6 months of age.

Feline acne is a problem seen in cats primarily involving the formation of blackheads accompanied by inflammation on the cat's chin and surrounding areas that can cause lesions, alopecia, and crusty sores. In many cases symptoms are mild and the disease does not require treatment. Mild cases will look like the cat has dirt on its chin, but the dirt will not brush off. More severe cases, however, may respond slowly to treatment and seriously detract from the health and appearance of the cat. Feline acne can affect cats of any age, sex or breed, although Persian cats are also likely to develop acne on the face and in the skin folds. This problem can happen once, be reoccurring, or even persistent throughout the cat's life.

Sebaceous glands are skin glands that produce oil and are mostly found in the skin of the chin, at the base of the tail, and in the eyelids, lips, prepuce, and scrotum. They are connected to hair follicles. In acne, the follicles become clogged with black sebaceous material, forming comedones (also known as blackheads). Comedones can become irritated, swollen, infected, and ultimately pustules. These may elicit itching and discomfort due to swelling and bacterial growth inside infected glands. Cats may continue to scratch and reopen wounds, allowing bacterial infections to grow worse. Bacterial folliculitis occurs when follicules become infected with "Staphylococcus aureus", and commonly associated with moderate-to-severe feline acne. Secondary fungal infections (species "malassezia") may also occur.

Other conditions that can cause similar-appearing conditions include skin mites, ringworm, yeast infection, or autoimmune diseases such as eosinophilic granuloma complex ("rodent ulcers"). These can be ruled out by a simple biopsy of affected cells.

Feline acne is one of the top five most common skin conditions that veterinarians treat.

Acne fulminans (also known as "acute febrile ulcerative acne") is a severe form of the skin disease, acne, which can occur after unsuccessful treatment for another form of acne, acne conglobata. The condition is thought to be an immunologically induced disease in which elevated level of testosterone causes a rise in sebum and population of "Propionibacterium acnes" bacteria. The increase in the amount of "P acnes" or related antigens may trigger the immunologic reaction in some individuals and lead to an occurrence of acne fulminans. In addition to testosterone, isotretinoin may also precipitate acne fulminans, possibly related to highly increased levels of "P acnes antigens" in the patient's immune system. Acne fulminans is a rare disease. Over the past several years, fewer cases of this disease have occurred, possibly because of earlier and better treatment of acne. Approximately 100 patients with acne fulminans have been described.

Steroid acne is an adverse reaction to corticosteroids, and presents as small, firm follicular papules on the forehead, cheeks, and chest. Steroid acne presents with monomorphous pink paupules, as well as comedones, which may be indistinguishable from those of acne vulgaris. Steroid acne is commonly associated with endogenous or exogenous sources of androgen, drug therapy, or diabetes and is less commonly associated with HIV infection or Hodgkin's disease.

A papulopustular condition is a condition composed of both papule and pustules.

Examples of papulopustular conditions include:

- Papulopustular rosacea

- Papulopustular acne

- Sycosis

Hidradenitis is any disease in which the histologic abnormality is primarily an inflammatory infiltrate around the eccrine glands. This group includes neutrophilic eccrine hidradenitis and recurrent palmoplantar hidradenitis.

It can also be defined more generally as an inflammation of sweat glands.

Hidradenitis suppurativa is a chronic cutaneous condition originally thought to be primarily characterized by suppurative inflammation of the apocrine sweat glands. However, recent evidence supports that the primary event is follicular hyperkeratosis and obstruction.

Inside the pore are sebaceous glands which produce sebum. When the outer layers of skin shed (as they do continuously), the dead skin cells left behind may become "glued" together by the sebum. This causes a blockage in the pore, especially when the skin becomes thicker at puberty. The sebaceous glands produce more sebum which builds up behind the blockage, and this sebum allows bacteria to grow, including the species "Staphylococcus aureus" and "Propionibacterium acnes", which causes inflammation and infection.