The majority of today's elliptical trainers have moving handle bars that allow the user to work the upper body and the lower body simultaneously. For this reason, they are often referred to as "cross-trainers." They are designed to provide very efficient cardiovascular exercise while practically eliminating pressure to the joints and back.
You can get an effective aerobic workout with both an elliptical machine and a treadmill. In general, you can let your fitness goals determine whether you choose an elliptical machine or a treadmill — or a different piece of exercise equipment.