I'm just going to quote from wikipedia.
"A cyst is a closed sac having a distinct membrane and developing abnormally in a cavity or structure of the body. Cysts may occur as a result of a developmental error in the embryo during pregnancy or they may be caused by infections. However, they sometimes arise spontaneously with no apparent cause. Cysts may often be dangerous as they may have negative effects on the nearby tissue. They may contain air, fluids, or semi-solid material."
It could have been a Sebaceous cyst to be more precise:
"A sebaceous cyst (also called epidermal cyst, keratin cyst, or epidermoid cyst) is a closed sac or cyst below the surface of the skin that fills with a fatty white, semi-solid material called sebum.
The scalp, ears, genitals, and face are common sites for sebaceous cysts, though they may occur anywhere on the body except the palms of the hands and soles of the feet. They are smooth to the touch, vary in size, and are generally round in shape."
If you really want to see pictures google has some.