There are really three catagories of exercise bikes now - upright; recumbent and Spinner. 

Upright bikes are the traditional design that most of us have seen or used at one time and are an excellent way to build legs muscles and develop your heart and lungs and increase circulation. 

Recumbent bikes allow the user to sit in a more reclined postion with back support. This is more comfortable for some people allowing less strain on the back. This style puts more empahsis on the back of the legs and is a good cardio exercise as well.

Spinners evolved out of Spinning classes. These bikes simulate a road bike and use a direct drive system that allows the user to stand up in the pedals to mimic riding up a hill. All Spinners are manually adjusted for tension control whereas most of the above bikes are noe controlled electronically and plug into a power outlet.