Recumbent bikes are best for people with back or wrist problems.  The workout is less strenuous, which is great for people who are out-of-shape. The recumbent bike develops the gluteals (butt) and hamstrings more than the quads, but quad development is needed to prevent arthritis of the knees.  If you bike outside, the upright can deal more easily with uneven surfaces and lets you can stand on the peddles to go up hills.  If you are purchasing the bike, note that the recumbent bike is more expensive and take up more space.  Read more about the Different Kinds of Bikes at About.com.