There is a nice little sub-forum up here in the breed section where you can read about all sorts of things pertaining to "Gaited Horses".
A gait is just a way of moving, classified by a specific pattern of footfalls.
Most horses have three main gaits: walk, trot, canter.
A "Gaited" horse, has more gaits than the usual three. Different breeds call these "extra" gaits, different things. For example, with Tennessee Walking horses, their most notable gait is the Running Walk. The walker folks can explain a bit more in detail.
Usually, these non-WTC gaits are very smooth, and easy riding, making them great horses for people with back problems, though I have heard that pacing (have you seen a Standardbred Pacer, for example) will jar you to the core.