The pace is an additional gait that Standardbreds have been bred for.  The pace involves the front and back legs on each side of the body moving back and forward in unison.  The pace is a lateral gait, as opposed to the diagonal gait of the trot in which the diagonal legs move in unison ie left fore and right hind.

The pace can actually be quite comfortable to ride (particularly in western or stock saddles), and as it is a ground covering gait that is easy for the horse, Standardbreds are becoming much more popular in endurance sports.