A woman's body prepares for pregnancy each month (it's really optimistic about her baby-making possibilities). The lining of her uterus, the endometrium, swells and thickens and gets all puffed up with red blood cells until one of her ovaries releases an egg.
If there's no sperm to fertilize the egg, the uterus sheds the lining (we call this a period) and her body gets ready to start the process of readying it for pregnancy again.
The uterus' specialized (endometrial) cells are responsible for the swelling and thickening of the lining, and they're very good at what they do.
It's a problem when uterine tissue ends up in the wrong place -- say, the lining of the pelvis, or the bowel -- because the cells keep doing their job. They grow and bleed, scarring and inflaming delicate places. They also cause incredibly painful periods, painful bowel movements, pain during and after sex -- and, often, infertility.
Researchers don't know how that tissue comes to be in the wrong place, but there is hope for women with endometriosis who want to get pregnant. Talk to your doctor about your options.