Cysts can form anywhere in the body, however ovarian cysts are specific to the ovaries.  Usually harmless and benign, they are fluid filled sacs which often form during ovulation.  Follicular cysts can occur when the sac (follicle) in which the egg grows, does not break open to release the egg, but continues to grow, thus forming a cyst.  This should normally just go away within a few months. A corpus luteum cyst is a little different. Once the egg is released from the sac, the follicle should shrink and prepare itself for the next egg. However, if the sac fails to shrink, and re-seals, fluids build up inside creating a cyst.  A corpus luteum cyst can just disappear on their own within a few weeks.  They can however grow quite large in size of up to or over 10cm wide.  Both follicular and Corpus luteum cysts are known as follicular cysts.

Other types of common ovarian cysts are Dermoid cysts which are cysts on the ovary that contains hair, fat and other tissue, and Endometriomas which occur when tissue that normally grows inside the uterus, develops outside the uterus and attaches to the ovary to form a cyst. There are of course a few less common and malignant ovarian cysts, but thankfully these are rare.