Very good answers, just thought I'd add a few notes
Clomid- the pill form of fertility drugs, the cheapest drug by far works by "tricking" your brain into producing follicles (fluid filled cyst that contains the egg). Works for some people.
Repronex, Gonal-F, Follistim- the injectible fertility drugs are made from the actual hormones(LH&FSH) that ripen the follicles. They go directly to work on your ovaries and are much stronger than Clomid. These meds will work for almost everyone to at least produce good follicles.
Ovidrel, Novarel- This is injectible HCG, the same hormone your body makes when you are pregnant. But when the shot is given to a woman with mature follicles, it forces ovulation to occur
Hope this helps.