A cyst is a pocket of tissue in the body that forms a sac filled with fluid, air, or other material. Think of it like a small balloon that grows somewhere it shouldn't be, usually just under the skin or inside an organ. Most cysts are harmless and feel like a smooth lump if you touch them.

Cysts can appear for many reasons. Sometimes a duct or gland gets blocked, and fluid builds up behind the blockage. Other times, cells grow abnormally and form a little enclosed space. You might get a cyst on your skin, in your mouth, or even in places you can't see, like your kidneys or liver.

The important thing to know is that most cysts aren't dangerous. Many people have small cysts they never even notice. Some cysts go away on their own, while others stay put without causing any trouble. If a cyst becomes painful, infected, or keeps growing, a doctor can remove it or drain the fluid inside.

Don't confuse a cyst with a tumor. While both are abnormal growths, a tumor is a solid mass of tissue, whereas a cyst is a sac that usually contains fluid.