It is a four-beat gait, where each hoof hits the ground independently of the others. In terms of how the legs move, gallop is actually very similar to canter, except that the opposite pair of legs do not hit the ground together.
The canter and gallop are both very fast gaits of the horse. When it comes to horse canter vs gallop, the gallop is faster and uses much more energy. In the wild, a horse will only gallop for as long as is necessary to escape from danger.