There are three electric bike classifications in the U.S. — Class 1, Class 2, and Class 3. The main difference between the levels is the top assisted speed and if you need to pedal to get the motor to kick in.
Class 1 ebikes only assist when you pedal and reach a top speed of 20mph. Meanwhile, Class 2 electric bikes have the same 20mph top speed, but have a throttle to power the bike without pedaling.
Class 3 bikes are allowed to reach a maximum speed of 28mph, but can only assist when you're pedaling.