I know even less about racing saddles than I do about racing bridles, so all the information in this post is based solely on one day's worth of observation at Arapahoe Park in Aurora, Colorado. It's entirely possible that you would see a different mix of saddles at a larger, single-breed race track.
The average racing saddle looks a bit like a scaled down English saddle with a long seat and very short flap.
Racing saddles come in all sorts of colors and color combination. Each jockey has his or her own saddle, and many of the saddles are personalized with their owner's initials.
Although all the saddles are considerably smaller than a regular riding saddle, there is some variation in size. This one (shown on a Quarter horse) is teeny-tiny.
The same horse and saddle, this picture really lets you see how small the saddle is.
This light blue saddle is on the other end of the size spectrum.
Here's the full body shot. This horse is also a Quarter Horse.
The saddles are fastened with two separate girths. The under girth attaches to billets under the flaps, just like a regular English saddle.
The overgirth is placed over the saddle's seat right behind the stirrup leathers.
The buckle goes under the horse's belly and the excess strap is turned over itself and secured under the overgirth's keepers.
Hope this was helpful!