Like the other girls said there are many of different symptoms. And depending on the person, they can be mild or severe. The symptoms that I have, are as follows:
irregular period
pain in ovaries (varies, sometimes it is so awful I cannot stand it, and sometimes it is very mild, and sometimes it doesn't hurt at all)
pain in various areas of the stomach (my whole life I have always slept on my stomach, but now, I cannot. If I push ever so lightly on any area of my stomach and lower sides it hurts.)
tubal pregnancies, infertility and ovarian cysts are all very possible if you have endometriosis (sp?) I hope this helped some.