A cyst is a pouch-like structure that can form in different parts of the body, such as the skin, bone, or even internal organs. It is filled with fluid, semi-solid material, or air, and is typically enclosed by a membrane or wall. Cysts can vary in size and can be caused by various factors, such as infections, blockages, or genetic conditions. They are usually benign and may not cause any symptoms, but in some cases, they can become enlarged or infected, requiring medical intervention.