The dog is a carnivorous mammal that was domesticated by humans thousands of years ago. It serves humans in a variety of ways—as a companion, hunter, herder, and protector, and as a draft animal. For the blind, dogs serve as guides. In some places, dog racing is a popular sport. Because of its loyalty, obedience, courage, and friendliness, the dog is often referred to as "man's best friend.”

Dogs have held a prominent place in mythology and literature. Dogs are mentioned in the Bible and in such historic works as The Odyssey. Prehistoric paintings of dogs have been discovered on the walls of some caves. Famous dogs from books, comic strips, television and motion pictures include Asta, Lassie, Rin-Tin-Tin, Benji, Toto, and Snoopy.

The domestic dog is Canis familiaris of the family Canidae, which also includes the coyote, jackal, and wolf. There are several species of wild dogs, including the dingo of Australia, the dhole of India, the raccoon dog of China, the hunting dog of South Africa, and the bush dog of South America.

Domestic dogs retain some wild instincts. This explains why some dogs chase moving objects, scavenge for food, and turn around several times before lying down, as their ancestors did to trample down high grass for a bed. Domestic dogs often sleep curled up with their tails over their faces, just as wild dogs do to protect their faces from the elements. Domestic dogs, like wild dogs, eat quickly and are protective of their food.

The smooth coat ChihuahuaThe smooth coat Chihuahua is a small domestic dog. .