A cyst is a sac-like structure that contains fluid. It can form in various parts of the body, such as the skin, ovaries, or kidneys. Cysts can vary in size and may be filled with different types of fluids, including pus, blood, or a clear liquid. They can be caused by a variety of factors, such as infections, blockages, or genetic conditions. Treatment for cysts may involve draining the fluid or surgical removal, depending on the size and location of the cyst.