Women have two ovaries, one on either side of the uterus. The ovaries — each about the size of an almond — produce eggs (ova) as well as the female sex hormones estrogen and progesterone. An ovarian tumor is a growth of abnormal cells that may be either noncancerous (benign) or cancerous (malignant). Although benign tumors are made up of abnormal cells, these cells don't spread to other body tissues (metastasize). Ovarian cancer cells metastasize in one of two ways. Generally, they spread directly to adjacent tissue or organs in the pelvis and abdomen. They can also spread through your bloodstream or lymph channels to other parts of your body.
Three basic types of ovarian tumors exist, designated by where they form in the ovary. They include:
The exact cause of ovarian cancer remains unknown. Some researchers believe it has to do with the tissue-repair process that follows the monthly release of an egg through a tiny tear in an ovarian follicle (ovulation) during a woman's reproductive years. The formation and division of new cells at the rupture site may set up a situation in which genetic errors occur. Others propose that the increased hormone levels before and during ovulation may stimulate the growth of abnormal cells.
Learn More >> Risk Factors
Source: Mayo Clinic Online (http://www.mayoclinic.com/health/ovarian-cancer/DS00293)