Cysts are sac-like structures that can develop in various parts of the body, including the skin. They are often filled with fluid, pus, or other materials. Skin cysts are generally benign and can arise due to blocked oil glands or hair follicles. They may appear as small lumps under the skin and can sometimes be tender or painful. Treatment may involve drainage, surgical removal, or medication, depending on the size, location, and discomfort they cause.