Ovarian cysts are fluid-filled sacs formed in the ovaries. There are a few different types, but the most common are follicular or luteal cysts, which are formed during ovulation when the egg is not properly released from the follicle and remains in the ovary, or when the corpus luteum does not break down and dissolve after release of the oocyte. These cysts often disappear in a matter of months, but more serious cysts may cause abdominal pain and bleeding.