I see that there are upright bikes and recumbent bikes, if not more, and I’m curious what the difference really is? Is one better than the other?

I was checking out exercise bikes at Gym Source and they have a lot to choose from, so maybe I can narrow things down a bit if I know the pros and cons of each type of bike. Is one more comfortable than the other?

I don’t know, I hope it’s more than just a gimmick.