
This horse’s name comes from its ability to sprint fast over distances of a quarter of a mile or less. It was originally – and still is - used as a quarter mile racer. The best quarter horses cover the 440 yards in 21 seconds or less, beating the Thoroughbred. Some have even reached speeds of 55 mph.
The breed was developed in the United States from several others, including the Thoroughbred, the Morgan and the American Saddlebred. They are noted for their calm temperament, and besides racing were used as stock horses. Today they are used mostly for rodeo competitions - cutting, roping and barrel racing.
The Quarter Horse is stocky, fairly short and muscular with a deep chest, and stands between 14.3 to 16 hands. A distinguishing feature of the breed is its large powerful hindquarters. You will often see photos of this horse with its hind end shown in the forefront.

In the 1600s English colonists called these tough equines Celebrated American Quarter Running Horses. The official breed registry was formed in 1940.
The horse has 16 recognized colors. The most usual is sorrel, and others include bay, black, brown, buckskin and chestnut. An unusual horse color also found is grullo, also known as blue dun.
The American Quarter Horse is the most popular United States breed, and the AQHA has 4 million American Quarter Horses registered worldwide.
Sources:
http://www.answers.com/topic/quarter-horse
http://www.aqha.com/association/who/thehorse.html
http://en.wikipedia.org/wiki/American_Quarter_Horse