Endometriosis is a fairly common disease - but it isn't well known because it's hard to diagnose. Not all women have symptoms - the most common being pain & infertility. The pain can be very severe; it can be generalized pelvic pain, pain while going to the washroom, pain during sex, lower back pain or localized pain (localized to where a lesion may be found). The symptoms aren't necessarily related to severity of the disease though. Unfortunately, the endometrial lesions need to be seen, and so laparoscopic surgery is necessary for diagnosis.
Surgery is also the only "cure" - but it isn't really a cure since endometriosis recurs in 50% of women 5 years after surgery. Hormone therapy (birth control, estrogen-replacement) will control the symptoms but only while taking the drug. Having your tubes tied should prevent endometriosis development, but if you know you have endometriosis then lesions have already formed (and the beginning stages will not always be visible for removal). So really - pain management with pain killers is often the only option.
Endometrial tissue lines the uterus & is shed every month during menstruation. This tissue is composed of endometrial cells - many capillaries supply the tissue for rapid monthly growth... which is where all the blood comes from.
Anyway, in endometriosis, endometrial cells travel up the fallopian tubes (this is called retrograde menstruation & is actually common in most women - why it results in endometriosis in some women isn't known), out into the peritoneal cavity & embed themselves... well, anywhere they can. On the ovaries, the outside of the fallopian tubes, the walls of the peritoneal cavity, the bowels, the bladder - anywhere. Once there, they can grow, induce capillary growth & ... cycle. Which usually means pain. Of course, these cells aren't supposed to be there, so inflammation usually results, often leading to bloating, pain & aggravating the disease.
Go check out Tia's post for more info!