Ovarian cancer describes a type of cancer that develops from cells of the ovaries - the two almond-shaped structures that are responsible for ovulation (release of a monthly ovum) and secrete the female reproductive hormones estrogen and progesterone. Sometimes, cancers that develop from the fallopian tubes (the structures that take up the ovum and transport it to the uterus) are also counted in this category. There are different types of ovarian cancers. The most common type, which is thought to account for about 90 % or more of ovarian cancers, is epithelial ovarian carcinoma (which can be subdivided in four main subtypes, serous carcinoma, endometrioid carcinoma, clear cell carcinoma and mucinous carcinoma). It develops from the epithelial cells forming the outer lining of the ovaries. Because epithelial tumors often do not exhibit clear symptoms until they are progressed to a more advanced stage, they tend to remain undiagnosed and untreated for long periods of time, thereby leading to decreased remission and increased death rates. A second type of ovarian cancers are ovarian stromal tumors (if these tumors are cancerous, they are also called stromal cell ovarian cancer). They grow from the stromal cells, a connective tissue located at the inside of the ovary, which lends the ovary stability and support. Because these tumors grow at the inside of the ovary, they often significantly interfere with the function of the ovary - including hormone production - which can lead to irregular vaginal bleeding (overproduction of estrogen) or excessive growth of facial and body hair (overproduction of testosterone). Because of these marked symptoms, stromal tumors are often diagnosed earlier, making treatment and recovery more successful, and reducing death rates. A third - but very rare -  type of ovarian cancer are malignant germ cell tumors. These tumors grow from the female germ cell, the ovum, and can affect younger women. One common form of germ cell tumor, which can be benign or malignant, is the teratoma. Because teratomas grow from the pluripotent germ cells of the ovum, a histology of the tumor after it is resected will often show a variety of different tissue types, including hair, teeth, skin, blood vessels and even eye and brain-like structures. Depending on whether or not the germ cell was properly released by the ovary, these tumors can occur either in the ovary, the Fallopian tube, or the uterus. Despite their uncommon origin and appearance, germ cell tumors are usually successfully removed and treated. 

Not all ovarian tumors are clearly cancerous, however. In fact, these tumors operate on a continuous spectrum, with some being clearly benign, some being clearly malignant, and others having a less clearly definable intermediate molecular makeup. Moreover, ovarian masses can also be benign fluid-filled cysts, which often resolve on their own and do not require any treatment at all.