A cyst is a closed sac-like structure that can form in various parts of the body. It is usually filled with fluid, air, or other substances. Cysts can vary in size and may be found on the skin, in organs, or even within bones. While many cysts are harmless and may not cause any symptoms, some can lead to discomfort or complications if they grow large or become infected. There are different types of cysts, such as sebaceous cysts, which occur in the skin, and ovarian cysts, which develop in the ovaries. Most cysts are benign, meaning they are not cancerous, but it's important to consult a healthcare professional if you notice any unusual lumps or changes in your body.