A dog breed is a particular strain of dog that was purposefully bred by humans to perform specific tasks, such as herding, hunting, and guarding. Dogs are the most variable mammal on Earth, with artificial selection producing around 450 globally recognized breeds. These breeds possess distinct traits related to morphology, which include body size, skull shape, tail phenotype, fur type, body shape, and coat colour. Their behavioral traits include guarding, herding, and hunting, and personality traits such as hyper-social behavior, boldness, and aggression. Most breeds were derived from small numbers of founders within the last 200 years. As a result, today dogs are the most abundant carnivore species and are dispersed around the world.

A dog breed will consistently produce the physical traits, movement and temperament that were developed over decades of selective breeding. For each breed they recognize, kennel clubs and breed registries usually maintain and publish a breed standard which is a written description of the ideal specimen of the breed. Other uses of the term breed when referring to dogs include pure breeds, cross-breeds, mixed breeds and natural breeds.

Prior to the standardization of dog breeds, there were different types of dogs that were defined by their function. Many different terms were used to describe dogs, such as breed, strain, type, kind, and variety. By the end of the Victorian era, society had changed and so did the role of dogs. Form was given a more prominent role than function. Different types or breeds of dog were being developed by breeders who wanted to define specific characteristics and desirable features in their dogs. Driven by dog shows and the groups that hosted them, the term dog breed took on an entirely new meaning. Dog show competitions included best-in-breed winners, and the purebreds were winning. Breed standards are the reason the breed came to be, and with those standards are key features, including form, function and fitness for purpose. The Kennel Club in the UK was founded in 1873, and was the world's first national kennel club and breed registry. The International Canine Federation was founded in 1911 as a worldwide organization. Its objective is to bring global uniformity to the breeding, exhibiting and judging of pure-bred dogs. It now has 99 member countries.