Thoroughbreds are built for speed. They are tall (16+hands), with long legs. They are very sleek. They are well muscled, but they don't look stocky like a Quarter Horse. They are most often chestnut, bay, gray and black. Their head is medium sized and is refined, you can see some of their Arabian decendents in their faces. Thoroughbreds are decedents of the Godolphin Arabian, the Byerly Turk and the Darley Arabian. These three stallions were bred with English mares, and are considered the originators of the Thoroughbred breed. Thoroughbreds are often used for racing, however they also do well in other English disciplines such as three day eventing, and hunter pleasure shows. Thoroughbreds tend to be more hot-headed than other breeds. They have a lot of energy and can be more difficult to control. However, training does wonders, and Thoroughbreds can be wonderful, sweet horses. To be a registered Thoroughbred with the Jockey Club, it must be bred through live cover, meaning the stallion and mare must be together. Source: grew up on a stables!

Basically it comes down to breed & pedigree. They are usually race horses & are often additionally bred for show jumping, dressage & polo. They are tall, slim & will rarely be white. They are often known to be 'hot blooded' & bred for their agility & speed & have been influential in the creation of many other breeds. http://en.wikipedia.org/wiki/Thoroughbred_horse#Terminology