There are two forms in which Standardbreds can race: Trotters or Pacers. The difference is in gait (speed and movement) in which the horses race.
Trotters are horses that race in a gait known as a trot. A trot is usually a moderate speed where the horse's legs move on diagonals (front right moves with back left and vice versa) in a two beat fashion, but in the case of Trotters the 'moderate' idea is thrown out the window. Although it is not as fast as a gallop, horses do hit some fairly high speeds.
Pacing is similar to trotting in the fact that it is also a two beat gate that is remarkably fast. However, in a pace the horses legs move together on one side (right front moves with right back). Pacers make up a good portion of harness racing and is generally more popular because of it's unique gait. It is also much faster than trotting, and more speed usually means more excitement.
To help show you what I mean (hopefully) here are some prime examples of harness racing.
The first picture is of a Pacer, while the second is of a Trotter.
Unlike Thoroughbreds and Quarter Horses, Standardbreds do not use a traditional starting gate. This is mainly because it would be near impossible to get the speed you want without breaking the proper gate. To solve this problem, a Standardbred starting gate moves with the horses. It is mounted onto the back of a truck, and as the truck is in motion, the horses get to their proper position and speed. The truck eventually gains more speed than the horses and the long gates fold around the truck giving the horses room to move.
Sadly, I have only seen Thoroughbred racing live. Although it is amazing and probably my favorite form of racing, I am really hoping to see the other three types.
I encourage everyone to watch different forms of racing and find which one they find the most interesting, Whether it be the classic a regal world of Thoroughbreds, the high speed battles of Quarter Horse racing, or the complex sport of Harness racing. No matter which branch or breed of horse racing you choose, you are almost always guaranteed great competition, amazing speed, a grand amount of excitement, and the ability to watch a magnificent animal doing what it loves...racing.