Galloping is the fastest of horse gaits; horses can achieve speeds of up to 30 miles an hour (50 kilometers an hour) at the gallop. Typically this gait can only be sustained for a short period of time, because it requires a huge amount of energy. Fans of horse racing have seen horses worked at the gallop, since this gait is classically used in racing, and galloping is also used in hunting and in the show ring. As a general rule, this is a gait for advanced riders only, because it is difficult to control a horse at the gallop.
This is a four beat gait in which the legs are extended as far as possible to cover ground. It is very similar to the three beat canter, except that the gallop requires even more power from the hindquarters. In nature, this gait undoubtedly evolved to allow horses to escape quickly from predators, which is why it relies on a burst of speed, rather than stamina. The goal is to get away from a dangerous location, rather than to keep up a sustained flight.
If you watch a video of a horse galloping in slow motion, you can see that the gallop includes a moment of suspension, in which all four legs are off the ground. During the moment of suspension, the legs are bent under the body of the horse, right before the horse lands a hind leg and takes off with explosive power, extending the front legs as far as possible to move forward.
Unlike the trot and canter, the gallop essentially only has one speed. Some show horses are taught to demonstrate the gallop in hand, a more controlled form of the gallop, but generally, horses are either galloping or cantering.
Sitting a gallop is tricky. Most riders adopt a two-point position, in which the body is raised slightly over the saddle and leaned forward over the horse for balance. It takes time to learn to sit this gait securely, and to control a horse at the gallop; an unbalanced rider or a rider who lacks control may find that a gallop turns into a sprint which cannot be controlled until the horse runs out of energy, and this is not desirable. As a general rule, riders should become very comfortable with the canter before pushing the gait into a gallop, and many riders try galloping on a lead line first, allowing someone else to control the horse while they learn to sit the gait securely.