A cyst is a sac-like thin tissue that can contain fluid, gas, or air. It can grow anywhere in the body or under the skin. It is usually painless and noncancerous. It can vary from microscopic size to large structures that can cause internal organ displacement.

Some of the causes of cysts are:
infections
blockages in the ducts of the body
genetically inherited diseases
chronic inflammation
a parasite
defect in the cells
faults during fetal development
an injury that can break a blood vessel

A cyst is a sac formed by an outer layer of abnormal cells, while an abscess is a pus-containing infection in the body due to bacteria, virus, or fungus. A cyst grows slowly and is usually painless. However, an abscess is painful, swollen, irritated, and red. The infection can result in symptoms in other parts of the body too. A cyst can sometimes become infected and turn into an abscess.

Some of the commonly found cysts are:
epidermoid cyst - occurs during trauma around a hair follicle in the skin
ganglion cyst - forms in any joints, for example, on the wrist
ovarian cyst - results in fluid build-up on or within the surface of the ovary
breast cyst - occurs due to fluid accumulation near the breast glands
chalazion - forms when the oil ducts in the eyelids are blocked

A biopsy is performed to determine if the cyst is benign or malignant. Most of the cysts go away on their own. If not, they are drained using a needle or surgically removed by a doctor. A corticosteroid injection or antibiotics might be given to decrease the inflammation of the cyst.