A cyst is a closed sac, having a distinct membrane and division compared with the nearby tissue. Hence, it is a cluster of cells that has grouped together to form a sac (not unlike the manner in which water molecules group together, forming a bubble); however, the distinguishing aspect of a cyst is that the cells forming the "shell" of such a sac are distinctly abnormal (in both appearance and behaviour) when compared with all surrounding cells for that given location. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, sometimes a cyst may resolve on its own. When a cyst fails to resolve, it may need to be removed surgically, but that would depend upon its type and location.

Cancer-related cysts are formed as a defense mechanism for the body, following the development of mutations that lead to an uncontrolled cellular division. Once that mutation has occurred, the affected cells divide incessantly (and become known as cancerous), forming a tumour. The body encapsulates those cells to try to prevent them from continuing their division and to try to contain the tumour, which becomes known as a cyst. That said, the cancerous cells still may mutate further and gain the ability to form their own blood vessels, from which they received nourishment before being contained. Once that happens, the capsule becomes useless and the tumour may advance from benign to a cancer.

Some cysts are neoplastic and thus, are called cystic tumors; many types are not neoplastic. Some are dysplastic or metaplastic. Pseudocysts are similar to cysts (having a sac filled with fluid), but lack an epithelial lining.

Radicular cysts are by far the most common cyst occurring in the jaws.

Treatment ranges from simple enucleation of the cyst to curettage to resection. There are cysts—e.g., buccal bifurcation cyst—that resolve on their own, in which just close observation may be employed, unless it is infected and symptomatic.

Two percent of women will have a Bartholin's gland cyst at some point in their lives. They occur at a rate of 0.55 per 1000 person-years and in women aged 35–50 years at a rate of 1.21 per 1000 person-years. The incidence of Bartholin duct cysts increases with age until menopause, and decreases thereafter. Hispanic women may be more often affected than white women and black women. The risk of developing a Bartholin's gland cyst increases with the number of childbirths.

Blocked sebaceous glands, swollen hair follicles, high levels of testosterone and the use of androgenic anabolic steroids will cause such cysts.

A case has been reported of a sebaceous cyst being caused by the human botfly.

Hereditary causes of sebaceous cysts include Gardner's syndrome and basal cell nevus syndrome.

Treatment for dermoid cyst is complete surgical removal, preferably in one piece and without any spillage of cyst contents. Marsupialization, a surgical technique often used to treat pilonidal cyst, is inappropriate for dermoid cyst due to the risk of malignancy.

The association of dermoid cysts with pregnancy has been increasingly reported. They usually present the dilemma of weighing the risks of surgery and anesthesia versus the risks of untreated adnexal mass. Most references state that it is more feasible to treat bilateral dermoid cysts of the ovaries discovered during pregnancy if they grow beyond 6 cm in diameter.

While Bartholin cysts can be quite painful, they are not life-threatening. New cysts cannot absolutely be prevented from forming, but surgical or laser removal of a cyst makes it less likely that a new one will form at the same site. Those with a cyst are more likely than those without a cyst to get one in the future. They can recur every few years or more frequently. Many women who have marsupialization done find that the recurrences may slow, but do not actually stop.

Cysts derived from CNS tissues are very common in America. They are a subtype of cerebrovascular diseases, which are the third leading cause of death in America. Generally, CNS cysts are present in all geographic regions, races, ages, and sexes. However, certain types of CNS cysts are more prevalent in certain types of individuals than others. Some examples of incidence rates in specific types of cysts include:

- Arachnoid cysts are more prevalent in males than females

- Colloid cysts are more prevalent in adults

- Dermoid cysts are more prevalent in children under 10 years of age

- Epidermoid cysts are more prevalent in middle-aged adults

The prognosis depends upon the type, size and location of a cyst. Most cysts are entirely benign, and some may require no treatment. Rarely, some cystic lesions represent locally aggressive tumors that may cause destruction of surrounding bone if left untreated. This type of cyst are usually removed with a margin of healthy bone to prevent recurrence of new cysts. If a cyst expands to a very large size, the mandible may be weakened such that a pathologic fracture occurs.

Many CNS cysts form in the womb during the first few weeks of development as a result of congenital defects. In adults cysts may also form due to a head injury or trauma, resulting in necrotic tissues (dead tissue), and can sometimes be associated with cancerous tumors or infection in the brain. However, the underlying reasons for cyst formation are still unknown.

About 90% of pilar cysts occur on the scalp, with the remaining sometimes occurring on the face, trunk and extremities. Pilar cysts are significantly more common in females, and a tendency to develop these cysts is often inherited in an autosomal dominant pattern. In most cases, multiple pilar cysts appear at once.

The clinical management of a cyst of Montgomery depends upon the symptoms of the patient.

If there are no signs of infection, a cyst of Montgomery can be observed, because more than 80% resolve spontaneously, over only a few months. However, in some cases, spontaneous resolution may take up two years. In such cases, a repeat ultrasonography may become necessary. If, however, the patient has signs of an infection, for example reddening (erythema), warmth, pain and tenderness, a treatment for mastitis can be initiated, which may include antibiotics and non-steroidal anti-inflammatory drugs (NSAIDs). With treatment, inflammatory changes usually disappear quickly. In rare cases, drainage may become necessary. A surgical treatment of a cyst of Montgomery, i.e. a resection, may become necessary only if a cyst of Montgomery persists, or the diagnosis is questioned clinically.

The prognosis seems to be excellent. In one series, all adolescent patients with a cyst of Montgomery had a favourable outcome.

Epidermoid cysts commonly result from implantation of epidermis into the dermis, as in trauma or surgery. They can also be caused by a blocked pore adjacent to a body piercing. They are also seen in Gardner's syndrome and Nevoid Basal Cell Carcinoma Syndrome on the head and neck. They can be infected by bacteria and form a pimple-like shape.

A dermoid cyst is a teratoma of a cystic nature that contains an array of developmentally mature, solid tissues. It frequently consists of skin, hair follicles, and sweat glands, while other commonly found components include clumps of long hair, pockets of sebum, blood, fat, bone, nails, teeth, eyes, cartilage, and thyroid tissue.

As dermoid cysts grow slowly and contain mature tissue, this type of cystic teratoma is nearly always benign. In those rare cases wherein the dermoid cyst is malignant, a squamous cell carcinoma usually develops in adults, while infants and children usually present with an endodermal sinus tumor.

The thyroglossal tract arises from the foramen cecum at the junction of the anterior two-thirds and posterior one-third of the tongue. Any part of the tract can persist, causing a sinus, fistula or cyst. Most fistulae are acquired following rupture or incision of the infected thyroglossal cyst. A thyroglossal cyst is lined by pseudostratified, ciliated columnar epithelium while a thyroglossal fistula is lined by columnar epithelium.

Nabothian cysts are considered harmless and usually disappear on their own, although some will persist indefinitely. Some women notice they appear and disappear in relation to their menstrual cycle. If a woman is not sure the anomaly she has found on her cervix is a nabothian cyst, a visit to a doctor is recommended to rule out other conditions.

Rarely, nabothian cysts have a correlation with chronic cervicitis, an inflammatory infection of the cervix.

Nabothian cysts are not considered problematic unless they grow very large and present secondary symptoms. A physician may wish to perform a colposcopy or biopsy on a nabothian cyst to check for cancer or other problems. Two methods for removing these cysts include electrocautery and cryofreezing, although new cysts may form after the procedure.

Although the treatment of the cyst was previously enuclation of the cyst with removal of the involved tooth or enuclation with root-canal treatment, the current management is enuclation with the preservation of the involved tooth. However, recent evidence suggests self-resolution of this type of cyst, thus close observation with meticulous oral hygiene measures can be employed unless the cyst is infected and symptomatic.

One proposed cause of pilonidal cysts is ingrown hair. Excessive sitting is thought to predispose people to the condition, as sitting increases pressure on the coccygeal region. Trauma is not believed to cause a pilonidal cyst; however, such an event may result in inflammation of an existing cyst. However, there are cases where this can occur months after a localized injury to the area. Some researchers have proposed that pilonidal cysts may be the result of a congenital pilonidal dimple. Excessive sweating can also contribute to the cause of a pilonidal cyst. Moisture can fill a stretched hair follicle, which helps create a low-oxygen environment that promotes the growth of anaerobic bacteria, often found in pilonidal cysts. The presence of bacteria and low oxygen levels hamper wound healing and exacerbate a forming pilonidal cyst.

Thyroglossal Duct Cysts are a birth defect. During embryonic development, the thyroid gland is being formed, beginning at the base of the tongue and moving towards the neck canal, known as the thyroglossal duct. Once the thyroid reaches its final position in the neck, the duct normally disappears. In some individuals, portions of the duct remain behind, leaving small pockets, known as cysts. During a person's life, these cyst pockets can fill with fluids and mucus, enlarging when infected, presenting the thyroglossal cyst.

A urachal cyst is a sinus remaining from the allantois during embryogenesis. It is a cyst which occurs in the remnants between the umbilicus and bladder. This is a type of cyst occurring in a persistent portion of the urachus, presenting as an extraperitoneal mass in the umbilical region. It is characterized by abdominal pain, and fever if infected. It may rupture, leading to peritonitis, or it may drain through the umbilicus. Urachal cysts are usually silent clinically until infection, calculi or adenocarcinoma develop.

Theca lutein cyst is a type of bilateral functional ovarian cyst filled with clear, straw-colored fluid. To be classified a functional cyst, the mass must reach a diameter of at least three centimeters.

These cysts result from exaggerated physiological stimulation (hyperreactio luteinalis) and are usually associated with markedly elevated levels of beta-human chorionic gonadotropin (beta-hCG). They are thus associated with gestational trophoblastic disease (molar pregnancy), diabetes mellitus, alloimmunisation to Rh-D, and multiple gestations. They have rarely been associated with chronic kidney disease (secondary to reduced hCG clearance) and hyperthyroidism (given the structural homology with TSH). These cysts resolve after pregnancy. Rarely, when the theca-lutein cysts are stimulated by gonadotropins, massive ascites can result. In most cases however, abdominal symptoms are minimal and restricted to peritoneal irritation from cyst hemorrhage. Surgical intervention may be required to remove ruptured or infarcted tissue.

Women who smoke have a twofold increase for functional cysts.

The nasopalatine duct cyst (abbreviated NPDC) occurs in the median of the palate, usually anterior to first molars. It often appears between the roots of the maxillary central incisors. Radiographically, it may often appear as a heart-shaped radiolucency. It is usually asymptomatic, but may sometimes produce an elevation in the anterior portion of the palate. It was first described by Meyer in 1914.

The median palatal cyst has recently been identified as a possible posterior version of the nasopalatine duct cyst.

A cyst of Montgomery is a benign breast mass, usually found in adolescent girls. Typically, it resolves spontaneously by itself.

The epidermoid cyst may have no symptoms, or it may hurt when touched. It can release pus. It is very common for women on the major or minor labia. In contrast to pilar cysts, epidermoid cysts are usually present on parts of the body with relatively little hair.

Occasionally, an epidermoid cyst will present with Trigeminal neuralgia.

Although they are not malignant, there are rare cases of malignant tumors arising from an epidermoid cyst.

Choledochal cysts are treated by surgical excision of the cyst with the formation of a roux-en-Y anastomosis hepaticojujenostomy/ choledochojujenostomy to the biliary duct.

Future complications include cholangitis and a 2% risk of malignancy, which may develop in any part of the biliary tree. A recent article published in Journal of Surgery suggested that choledochal cysts could also be treated with single-incision laparoscopic hepaticojejunostomy with comparable results and less scarring. In cases of saccular type of cyst, excision and placement of T-shaped tube is done.

Currently, there is no accepted indication for fetal intervention in the management of prenatally suspected choledochal cysts.