The dog has developed into hundreds of breeds with a great degree of variation. For example, heights at the withers ( the highest point on an animal's back, on the ridge between its shoulder blades) range from just a few inches (such as the Chihuahua) to roughly three feet (such as the Irish Wolfhound), and colors range from white to black, with reds, grays (usually called blue), and browns occurring in a tremendous variation of patterns.

Dogs, like humans, are highly social animals and this similarity in their overall behavioral pattern accounts for their trainability, playfulness, and ability to fit into human households and social situations. Dogs fill a variety of roles in human society and are often trained as working dogs. For dogs that do not have traditional jobs, a wide range of dog sports provide the opportunity to exhibit their natural skills. In many countries, the most common and perhaps most important role of dogs is as companions.