The canter is a controlled gait, generally English. The western version of a canter, with the same footfall pattern, although generally slightly slower, is the lope.
Unlike the walk and the trot, which do not have a "direction", the canter has two "leads" that can be taken. The "lead" is the order the hoof beats fall, either "right lead" or "left lead". (As in the photo above) The left lead begins with the right hind hoof first, then the left hind and right front simultaneously, and finally the right front. The Right lead is the opposite with the left hind first, the right hind and left front next, and the right front last.
You can tell which lead the horse is on by watching the horses shoulders. The right front, or left front will appear to stride out in front, telling you which lead the horse is on.