Both anti-aromatase agents. Proviron does an OK job. Arimidex does it damn near perfectly.
Medically? Proviron is used to treat male-impotence by creating an androgenic environment, and by potentially supressing some of the offending estrogen via its anti-aromatase properties.
Arimidex, like any of the other anti-estrogens or anti-As, is used to treat cases of breast cancer and osteoporosis in women, or any other aggravating symptoms that estrogen is responsible for. These drugs are most commonly used in post-menopausal women. Excess estrogen can cause some very real problems in either gender, but yes, arimidex was in fact made for medical use in women.
Ask one of the chemists on the board if you want a chemical explanation. Frankly, I'm not sure why you asked this, or why I just spent seven minutes answering it, but you could redeem us both if you just tell me my info helped.
-e