Link to Pub Med's access to medical abstracts. These are fluid-filled bags or sacs, usually very small although they can become quite large, which are within a woman's ovaries.
All ovarian cysts contain fluid to some degree.
The commonest one is due to what is known as a follicular cyst. A follicle is a normal part of egg development, and is the fluid-filed sac that contains an egg. Very occasionally, ovarian tissue grows abnormally to form cysts with other body tissue in them such as hair or teeth. These are known as dermoid cysts. Most women with ovarian cysts have no symptoms at all.