Endometriosis is the term that reflects the abnormality of endometrium. It is a tissue that lines the inside of the uterus and it sheds along with the unfertilized egg during every menstrual period. But in case of endometriosis, this tissue is found outside of the uterus like on the ovaries, fallopian tubes or other parts of the pelvic cavity. This leads to symptoms like painful menstruation and dyspareunia (painful sex).