Cysts are closed sac-like capsule structures that are often filled with a semisolid keratinous material. Sebaceous cysts are the most common type requiring excision. They form when a pocket under the skin blocks the natural sloughing off cells to the surface.
A cyst begins small but can grow to a large size as more and more cells become trapped. Often, cysts are removed by an incision that is made in the skin overlying the cyst.