Endometriosis ("endo" for short) is a chronic disease in which tissue like the endometrium (which is the inner lining of the uterus) appears outside of the uterus, usually in the pelvic area, like on the outside of the uterus, or around the bowel. Now, if that's not strange enough, what happens is that this misplaced tissue follows your menstrual cycle: it grows before your period, and breaks down a bleeds during your period. So basically you are suffering from monthly internal bleeding, which causes all sorts of symptoms.
Women with endometriosis also often suffer from allergies and chemical sensitivities.
Because these symptoms are so varied, and could be due to many different causes, endo often goes undiagnosed. Women are often told that their symptoms are all in their head, or just a normal part of having a period.
The only sure way to diagnose endo is to actually see the growths, and this is usually done using laparoscopy, a minor surgical procedure involving a tiny camera. Once the location and extent of the growth is known, you and your doctor can discuss treatment options.
If you want to know more about endometriosis, please leave my site and go visit people who know a lot more than I do. Start with the Endometriosis Association International Headquarters.