Dogs and humans exist in a mutualistic relationship that dates back thousands of years. Dogs serve humans in many ways. There are guard dogs, hunting dogs, and shepherd dogs. Relationships between humans and dogs are often characterized by strong emotional bonds, which run both ways. Consequently, dogs are very popular as pets and companions, independent of any utilitarian considerations.
Dog societies are characterized by companionate hierarchy, in which each individual has a rank in society, and in which there is intense loyalty within the group. Dogs thrive in human society because their relationships with humans mimic their natural social patterns. The dog is always aware of its rank vis-a-vis other individuals in the group, and it may be noted that an assertive dog often considers itself the alpha animal, while considering its human owner to be subordinate.
There are numerous breeds of dog, many of which evolved under a process of artificial selection. Because of this, some breeds are highly specialized, and there is extraordinary morphological diversity across different breeds. Despite these differences, dogs are able to distinguish dogs from other kinds of animal.
The definition of a breed is a matter of some controversy. Some groups use a definition that ultimately requires extreme in-breeding to qualify. Dogs that are bred in this manner often end up with severe health problems. Other organizations define a breed more loosely, such that an individual may be considered of one breed as long as, say, three of its grandparents were of that breed. These considerations come into play among breeders who enter their dogs in dog show competitions.