Ellipticals can be categorized as rear-drive, front-drive, or center drive.  Rear-drive machines have the axis of movement in the back of the machine. You typically have to step over the foot pedals to get on the machine and it will feel like a normal walking or running motion. Front-drive machines will have an axis in the front. They will be easier to step onto and will feel more like a hill climb or a stair stepper.  Center drive machines have an axis in the center.  This is the most recent elliptical design.