Medscape Today defines endometriosis as:

Endometriosis is a condition in which tissue resembling endometrial glands or stroma occurs outside the uterus. Endometriosis lesions are most often found in the pelvis. Common sites are the peritoneal surfaces posterior to the uterus; the ovary; the peritoneal surfaces anterior to the uterus; the bowel; the bladder; and the appendix. Rarely, endometriosis lesions occur at sites outside the pelvis, such as the respiratory diaphragm.

Endometriosis is not only painful and dangerous because of the heavy bleeding, it can also lead to infertility. The good news? Surgery can help. . The Science Blog reports:

One of the few prospective studies of surgical treatment for endometriosis has shown that the majority of women who undergo surgery for the condition experience dramatic improvement in pain and discomfort.

The study also showed that women who had previously given birth were twice as likely to get pregnant again following surgery than women who had not, and that increasing age reduced the likelihood of becoming pregnant.

The Infertility Blog explains further:

No one really knows what causes endometriosis. The prevailing theory is that it is caused by menstrual blood that backs up through the tubes during one's period. This blood also contains the shedding glands from the lining of the uterus (the endometrium). It's these glands that start the problem. In some women, for unknown reasons, these glands stick and continue to grow in areas like the ovary (most common area), the tubes and other areas in the pelvis near the bladder and rectum. These growing glands, only in some cases, can cause scarring and inflammation that lead to tubal blockage and pain. Ten percent of women have endometriosis.