Cantering is a three beat horse gait which falls between the trot and the gallop in terms of speed. This gait is very smooth, especially on a well-trained horse, and developing skills at the canter is important for both horses and riders. When the canter is accelerated, it turns into the gallop, a four beat and extremely fast gait.
Describing a horse's movements at the canter is a bit complicated; basically, the gait starts with one rear leg on the ground, and the three other legs in the air. The next step involves setting the opposite foreleg and the other hind leg on the ground, and then the original leg is lifted while the foreleg on the same side is brought to the ground. Then, all the legs are lifted at once to prepare for the next cycle, in what is known as the “suspension phase.” While riders had long suspected that there was a point in the canter where no feet were touching the ground, it was Edward Muybridge who was able to prove it in the 1800s, with the assistance of photography.
You may have heard riders referencing “leads” when discussing cantering. The lead is defined by the leg which leaves the ground last before the suspension phase, and it makes a big difference to the horse's appearance and balance. Horses need to be able to change leads easily to compensate for changes in terrain, and in dressage, horses are sometimes asked for flying lead changes, in which the lead is changed in mid-canter.
As with other horse gaits, there are different types of cantering. All horses can naturally demonstrate the working canter, which is a natural form of the gait. In a collected canter, the horse is asked for more control, creating a compact gait with smaller strides which showcases control and power in the hindquarters. Horses can also be taught to do the extended canter, a gait which is very close to a gallop. Western horses often learn to lope; loping is a slower version of cantering well-suited to a long workday on the range.
Many riders like to sit while cantering, because it offers them greater control. While seated, a rider can shorten or lengthen the stride, and keep the horse stable. Some riders use a half-seat, in which they are partially raised from the saddle; this is common in jumping. Other riders use a version of posting, in which they move from a seated to a standing position; this is sometimes seen in polo and buzkashi.