There are numerous dog breeds with over 800 being recognized by various kennel clubs worldwide. Many dogs, especially outside the United States and Western Europe, belong to no recognized breed. A few basic breed types have evolved gradually during the domesticated dogs relationship with humans over the last 10,000 or more years, but all modern breed are of relatively recent deviation. Many of these are the product of a deliberate process of artificial election. Because of this some breed are highly specialized, and there is extraordinary morphological diversity across different breeds. Despite these diversities, dogs are able to distinguish dogs from other kinds of animals.

Mixed-breed dogs or Mongrels (also called "mutts") are dogs that do not belong to specific breeds, being mixtures more than two in variant percentages. Mixed breed dogs and purebred dogs are both suitable as companions, pets, working dogs, or competitors in dog sports. Sometimes different breed dogs are deliberately bred to create crossbreeds such as the Cockapoo, a mixture of Cocker Spaniel and Miniature Poodle. Such deliberate crosses may display some degrees of hybrid vigor and other desirable traits, but may or may not inherit any of the desired traits of their parents, such as temperament or a particular coat color.

Modern dog breed show more variation in size, appearance, and behavior than any other domestic animal. Within the range of extremes dogs generally share attributes with their wild ancestors, the wolves. Dogs are predators and scavengers, possessing sharp teeth and strong jaws for attaching, holding, and tearing their food. Although selective breeding has changed the appearance of many breeds, all dogs retain basic traits from the distant ancestors. Like many other predatory mammals, the dog has powerful muscles, fused with bones, a cardiovascular system that supports both sprinting and endurance. Compared to the bone structure of the human foot, technically, dogs walk on their toes.

Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans.

Dogs detect sounds as low as the 16 - 20 Hz frequency range (compared to 20 - 70 Hz range for humans) and above 45 kHz (compared to 13 - 20 kHz for humans) and in addition, have a degree of ear mobility that helps them to rapidly pinpoint the exact location of a sound. Eighteen or more muscles can tilt, rotate and raise or lower a dog's ear. Additionally a dog can identify a sound's location much faster than a human can, as well as hear sounds up to four times the distance that humans are able to hear.

What information a dog actually detects when he is scenting is not perfectly understood. It now seems to be well established that dogs can distinguish two different types of scents: an air scent from some person or thing that has recently passed by, and a ground scent that remains detectable for a much longer period of time.

Dogs are very social animals, but their personality and behavior vary with breed as well as with how they are treated by their owners and others who come in contact with them. It is not uncommon for dogs to attack humans and other animals, however, this is usually because of lack of care of improper upbringing by its owner. Dogs are valued for their intelligence. This intelligence is expressed differently with different breeds and individual however.

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