Based on the link Greg posted, it looks like the main difference between a motor scooter and a moped is that a scooter can go nearly as fast as a car (up to around 95 kilometers per hour), whereas a moped has a small engine and can only go about 45 kilometers per hour. (A motorcycle does not have a step in the center. A motorcycle has a solid center, and you put your feet on either side when you ride, not in front of you.)