Dog breeds are groups of closely related and visibly similar domestic dogs with characteristic traits that are selected and maintained by humans, bred from a known foundation stock. A dog breed reproduces with young that are closely similar in appearance and other attributes to other dogs of the same breed.

A dog breed is represented by a sufficient number of individuals to stably transfer its specific characteristics over generations. Purebred 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. An individual dog is defined 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.

History of Dog Breeds
Aboriginal groups or pariah dogs establish themselves near human population, and further develop and maintain themselves without further selection. Neither they carry any specialized working dog functions. Working, hunting and other functional breeds most likely appeared when there is a demand for certain traits that are prevalent to the extension of the point one can devote his time and efforts to establish and maintain the group of dogs that perfect in certain traits valuable for that individual.

Initially the selections would have centered on domestication and useful behavior such as barking at strange creatures, livestock guarding or hunting ability. Some dog breeds, such as Saluki or New Guinea Singing Dogs , have been bred for specific characteristics for thousands of years. Some working dog breeds such as German Shepherds or Labrador Retriever  are established for hundreds years. Later, dogs were also selected for attractive and distinctive forms, resulting in a vast variety of different breeds. Similar dog breeds are classified by dog registries in Dog Breeds Groups.

At a certain point of dog breed development, group of individuals that have dogs of the same breed unite into a National Breed club, describe their dogs in a specific language by writing a breed standard. They describe the most desirable breed specimen and also specify working abilities, as well as undesirable traits for purebred dogs that can belong to this group of dogs. National Breed Clubs promote dog breeds by joining a local popular dog breed registry, as well as internationally, by joining international organizations. Many traditional dog breeds are recognized by the main breed registries are said to be "purebred".

Development of dog breeds
For the history and development of the dog, see Origin of the domestic dog, Ancient dog breeds, and Dog type.

There is much speculation but little evidence about why canids came to live with or near humans, possibly as long as 100,000 years ago. With the beginnings of agriculture around 12,000 years ago, humans began making use of dogs in various ways, resulting in physical differences between dogs and their wolf ancestors. In earlier times, little was written about dogs, although there were known dog types or landrace dogs, which developed over time with minimal human intervention, to fit in with the environment (including human culture) in which the dogs lived or live. Dog breeds in the modern sense date only to the accurate documenting of pedigrees with the establishment of the English Kennel Club in 1873, in imitation of other stud book registries for cattle and horses.

Many dog breeds today have names of original landrace types, such as the Border Collie. Other landrace types, such as retrievers, have been made more uniform in appearance through selective breeding, and developed into a variety of distinctive breeds. Varieties of purebred dogs kept for working purposes can vary in appearance from purebred dogs of the same breed kept as showdogs and pets.

New dog breeds are being continually created. They are either accidentally or purposely crossbred from existing breeds, developed for a specific style of work, or created just for marketing purposes. Recently discovered semi-feral and landrace types such as the New Guinea Singing Dog have been documented and registered as breeds for purposes of preservation. The Canadian department of agriculture has strict standards for the documenting of what it calls "emerging breeds". Many registries which require minimal documentation are available for registering new and existing breeds of dog. In general, a dog can only be guaranteed to be of a specific breed if it is documented in the stud book of a major dog registry or breed registry

From: wikipedia.org