What parts of your body benefit while bike riding?

What parts of your body benefit while bike riding?
I'm looking to slim down my thighs a bit and I'm not sure if bike riding helps the thigh area at all.

The bike certainly helps the thigh area, but also the calfs and your waist. If you are thinking of using a still bike where you pedal and move arm bars too - that helps your arms. Your bum could also be at benefit

If you're trying to build muscle in the legs and lose fat, your best bet is off-road bike riding, like mountain biking in hilly areas. Biking by itself is great cardiovascular exercise, but adding hills adds resistance, which works your leg muscles and ultimately burns fat. Riding bikes also has the benefit of being much easier on your joints than jogging.

Heart, lungs, as*, legs, stomach.

Biking will build muscle. If your are trying to slim down (meaning reduce body fat?), the aerobic benefits of biking will outweigh the increase in muscle mass that you will have, as far as appearance of your thighs is concerned.