Dog breeds are groups of closely related and visibly similar domestic dogs, which are all of the subspecies Canis lupus familiaris, having characteristic traits that are selected and maintained by humans, bred from a known foundation stock. The term dog breed is also used to refer to natural breeds or landraces, which arose through time in response to a particular environment that included humans, with little or no selective breeding by humans. Such breeds are undocumented, and are identified by their appearance and often by a style of working. Ancient dog breeds are some of the modern (documented) descendants of such natural breeds.

Dog breeds are not scientifically defined biological classifications, but rather are groupings defined by clubs of hobbyists called breed clubs. A dog breed is represented by a sufficient number of individuals to stably transfer its specific characteristics over generations. Dogs of same breed have similar characteristics of appearance and behavior, primarily because they come from a select set of ancestors who had the same characteristics. Dogs of a specific breed breed true, producing young closely similar to the parents. An individual dog is identified as a member of a breed through proof of ancestry, using genetic analysis or written records of ancestry. Without such proof, identification of a specific breed is not reliable. Such records, called stud books, may be maintained by individuals, clubs, or other organizations.

The domestic dog (Canis lupus familiaris), is a subspecies of the gray wolf (Canis lupus), a member of the Canidae family of the mammilian order Carnivora. The term "domestic dog" is generally used for both domesticated and feral varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in human history. The word "dog" may also mean the male of a canine species, as opposed to the word "bitch" for the female of the species.

The present lineage of dogs was domesticated from gray wolves about 15,000 years ago. Remains of domesticated dogs have been found in Siberia and Belgium from about 33,000 years ago. None of these early domestication lineages seem to have survived the Last Glacial Maximum. Although mDNA suggest a split between dogs and wolves around 100,000 years ago no specimens predate 33,000 years ago that are clearly morphologically domesticated dog.

Dogs' value to early human hunter-gatherers led to them quickly becoming ubiquitous across world cultures. Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and military, companionship, and, more recently, aiding handicapped individuals. This impact on human society has given them the nickname "Man's Best Friend" in the Western world. In some cultures, dogs are also source of meat. In 2001, there were estimated to be 400 million dogs in the world.

A breed is a group of domestic animals or plants with a homogeneous appearance, behavior, and other characteristics that distinguish it from other animals or plants of the same species, and arrived at through selective breeding. Despite the centrality of the idea of "breeds" to animal husbandry, there is no scientifically accepted definition of the term. A breed is therefore not an objective or biologically verifiable classification, but instead a term of art amongst groups of breeders who share a consensus around what qualities make some members of a given species members of a nameable subset. The term is distinguished from landrace, which refers to a naturally-occurring regional variety of domestic (and sometimes feral) animal through uncontrolled breeding.

When bred together, animals of the same breed pass on these predictable traits to their offspring, and this ability—known as "breeding true"—is a requirement for a breed. Plant breeds are more commonly known as cultivars. The offspring produced as a result of breeding animals of one breed with other animals of another breed are known as crossbreeds or mixed breeds. Crosses between animal or plant variants above the level of breed/cultivar (species, subspecies, botanical variety, even different genera) are referred to as hybrids.