A cyst is a sac-like pocket of membranous tissue that contains fluid, air, or other substances. Cysts can grow almost anywhere in the body or under the skin. There are many different types of cysts. Most cysts are benign (non-cancerous). Whether a cyst needs treatment depends on a number of factors, including the type of cyst, the location, if the cyst is causing pain or discomfort, and whether the cyst is infected.