What is the difference between a recumbent and an upright stationary bike?

Upright Bike

An upright bike is exactly like it sounds...upright. You are sitting on the exercise machine exactly like you would a normal bicycle in the upright position. Your seat is positioned almost directly vertical from the pedals, and you lean forward to grab the handle bars. For an upright exercise bike, though you are stationary, the motion is exactly the same as it would be for a real bike.

Recumbent Bike

A recumbent stationary bike positions you with your feet in front of, instead of beneath, your seat. Your seat has a backrest, unlike a real bike. The motion of your legs when pushing against the resistance is to push forward, not down. You are essentially leaning back, holding on to handles at your sides.

One of the big advantages of upright bikes over recumbent machines is the "footprint". Uprights occupy much less space on the floor.