Chickens are classified according to class, breed, and variety. Most classes are named for the country or region in which they were developed. Breeds are distinguished chiefly by the shape and weight of the body, and by habits. Varieties within a breed are distinguished by type of comb, and by color and pattern of plumage.

In each class, miniature chickens, called bantams, have been developed. Some poultry producers classify bantams as breeds; others consider them varieties. A bantam is about one-fourth the size of individuals of the large breed from which it was derived. Bantams are raised primarily as pets, or for show.

The four important classes of chickens raised in the United States are:

English Class, developed primarily for meat. Mature birds weigh 6 1/2 to 10 pounds (2.9 to 4.5 kg). Their shanks are clean. Most breeds have white skin and lay white-shelled eggs. Breeds important in the United States are Orpington, Cornish, and Australorp. Australorp was developed in Australia from the Orpington. Other breeds are Dorking, Sussex, and Redcap.

Asiatic Class, developed for meat and show. They have heavily feathered shanks. Mature birds weigh 7 to 11 pounds (3.2 to 5 kg). They lay brown-shelled eggs. Breeds include Brahma, Cochin, and Langshan. One variety—the Black Langshan—has white skin; the others, yellow. Several birds in the American and English classes were developed from birds of this class.

Mediterranean Class, developed primarily for egg production. Mature birds weigh 4 1/2 to 9 pounds (2 to 4 kg). They lay white-shelled eggs, and have clean shanks. The Leghorn, Minorca, and Ancona are important in the United States. The White Leghorn is the variety preferred by commercial egg producers. Other breeds are Spanish, Blue Andalusian, Buttercup, and Catalana. Leghorns and Anconas have yellow skin; others have white.

American Class, developed (with one exception) in the United States and Canada for both meat and egg production. Most mature birds weigh 7 1/2 to 9 1/2 pounds (3.4 to 4.3 kg). They have clean (unfeathered) shanks, and yellow skin. They lay brown-shelled eggs. Leading breeds are Plymouth Rock, Rhode Island Red, New Hampshire, Wyandotte, and Rhode Island White. Others are Dominique, Chantecler, Lamona, Buckeye, Holland, Delaware, Jersey Giant, and Java. The Java was developed in the East Indies. The Jersey Giant weighs 10 to 13 pounds (4.5 to 5.9 kg). The Rhode Island Red is the state bird of Rhode Island.

By crossing one breed or variety with another, poultry breeders produce hybrids that possess economically desirable qualities, for example, poultry meat is popularly spoken of as white meat (breast and wings) and dark meat (back and legs). Poultry has been bred that has proportionately larger breasts, thereby yielding larger proportions of white meat.

The domestic chicken is Gallus gallus of the pheasant family, Phasianidae.