PCOS is a condition in which a woman’s ovaries and, in some cases the adrenal glands, produce more androgens (a type of hormone) than normal.  High levels of these hormones interfere with the development and release of eggs as part of ovulation.  As a result, fluid-filled sacs or cysts can develop on the ovaries.

Because women with PCOS do not release eggs during ovulation, PCOS is the most common cause of female infertility.

A woman's ovaries have follicles, which are tiny, fluid-filled sacs that hold the eggs. When an egg is mature, the follicle breaks open to release the egg so it can travel to the uterus for fertilization. In women with PCOS, immature follicles bunch together to form large cysts or lumps. The eggs mature within the bunched follicles, but the follicles don't break open to release them.

As a result, women with PCOS often have menstrual irregularities, such as amenorrhea (they don’t get menstrual periods) or oligomenorrhea (they only have periods now and then). Because the eggs are not released, most women with PCOS have trouble getting pregnant.

References

http://www.nichd.nih.gov/health/topics/Polycystic_Ovary_Syndrome.cfm