To me, reining is the western form of dressage. They are very different, but also very alike. The elite among them compete with little to no visible cues, and these horses are true athletes. You cannot tell me there is no thrill in competing, and just performing the manuvers. It's amazing.
The differences are that reining is preferred on a loose rein, with the horses head in a natural position, though that is becoming somewhat corrupted. Ugh. But anyway, dressage has more contact between horse and the riders hands. Dressage saddles are also closer to the horse.
Sorry for the suckish reply, I can't type tonight apparently and I am super tired. =)
To rein a horse is not only to guide him,
but also to control his every movement.