The concept of purebred bloodstock and a controlled, written breed registry has come to be particularly significant and important in modern times. Sometimes purebred horses are incorrectly or inaccurately called "thoroughbreds". Thoroughbred is a specific breed of horse, while a "purebred" is a horse (or any other animal) with a defined pedigree recognized by a breed registry. Horse breeds are groups of horses with distinctive characteristics that are transmitted consistently to their offspring, such as conformation, color, performance ability, or disposition.

These inherited traits result from a combination of natural crosses and artificial selection methods. Horses have been selectively bred since their domestication. An early example of people who practiced selective horse breeding were the Bedouin, who had a reputation for careful practices, keeping extensive pedigrees of their Arabian horses and placing great value upon pure bloodlines. These pedigrees were originally transmitted via an oral tradition. In the 14th century, Carthusian monks of southern Spain kept meticulous pedigrees of bloodstock lineages still found today in the Andalusian horse.

Breeds developed due to a need for "form to function", the necessity to develop certain characteristics in order to perform a particular type of work. Thus, a powerful but refined breed such as the Andalusian developed as riding horses with an aptitude for dressage. Heavy draft horses developed out of a need to perform demanding farm work and pull heavy wagons. Other horse breeds developed specifically for light agricultural work, carriage and road work, various sport disciplines, or simply as pets.

Some breeds developed through centuries of crossing other breeds, while others descended from a single foundation sire, or other limited or restricted foundation bloodstock. One of the earliest formal registries was General Stud Book for Thoroughbreds, which began in 1791 and traced back to the foundation bloodstock for the breed. There are more than 300 horse breeds in the world today.