What Is a Kidney Cyst? There are different kinds of kidney cysts. A simple kidney cyst is a round pouch of smooth, thin-walled tissue or a closed pocket that is usually filled with fluid.

Boils and cysts are two types of growths on the body. Also known as furuncles, boils are pockets of pus (a creamy, white substance) on the skin caused by bacteria infecting hair follicles.