An ovarian cyst is a fluid-filled sac (follicle) that contains a maturing egg; it forms on the ovary's surface, then disappears not long after it releases the egg. If a mature egg is not released or if the sac reseals after the egg is released, the sac can swell up with fluid, forming a functional ovarian cyst.
Most functional ovarian cysts do not cause symptoms and will go away by themselves. Some may rupture, twist, or bleed, which can cause severe pelvic pain.
WebMD Medical Reference from Healthwise