NOTE: I am not a doctor nor do I have any other kind of medical training. All information here is simply the way I understand it through my own research and talking to doctors. Due to this there is the possibility some of what I've said could be wrong. If you notice any mistakes please let me know. If you think you have PCOS I encourage you to do your own research and talk to your doctor.
Polycystic Ovarian Syndrome is at it's root a hormonal imbalance. For reasons unknown women with PCOS produce more androgens (male hormones) than they should. This causes problems such as male pattern hair loss, hair growth in unwanted places, acne, ovarian cysts, infertility, and irregular periods.
The cysts, infertility, and irregular menstrual cycles are caused by the body not ovulating regularly (or at all). Each cycle follicles emerge on the ovaries containing immature eggs. Normally these eggs mature throughout the cycle, and when one matures completely it bursts through the follicle and begins it's journey down the Fallopian tube. This causes a hormone to be released letting the other eggs know they don't need to finish maturing and they are either discarded as waste or absorbed back into the body. At this point the woman will either get pregnant or ~14 days later begin her period.
For women with PCOS often the only step that is taken is the first. The eggs never finish maturing, and thus the body does not ovulate. Without ovulating it is impossible to get pregnant and the body will not begin menstruating. Instead of the follicles being discarded or absorbed they stay on the ovaries and become cysts.
Another issue women with PCOS generally have is insulin resistance. IR is similar to diabetes, only instead of the body not producing enough insulin the body has trouble absorbing it. The body will continue to produce insulin until it absorbs the proper amount, which means women with IR end up with a bunch of insulin left over. Through some process I don't quite understand this extra insulin is converted to fat. Because of this weight gain is a very common side effect of PCOS. Since the normal calories in - calories out equation does not work, it can also be extremely difficult to lose weight.
Not all women with PCOS will have the same symptoms. Since there is no definitive test it is often diagnosed through a process of exclusion.
