Pet

A pet is an animal kept for companionship and enjoyment or a household animal, as opposed to livestock, laboratory animals, working animals or sport animals, which are kept for economic reasons. The most popular pets are noted for their loyal or playful characteristics, for their attractive appearance, or for their song. Pets also generally seem to provide their owners with non-trivial health benefits; keeping pets has been shown to help relieve stress to those who like having animals around. There is now a medically-approved class of "therapy animals," mostly dogs, who are brought to visit confined humans. Walking a dog can provide both the owner and the dog with exercise, fresh air, and social interaction.

Pet popularity
Around 63 percent of all U.S. households (71.1 million) are pet owners, and more than half of these households have more than one animal. The two most popular pets in most Western countries have been cats and dogs. In the United States, a 2007-2008 survey shows that dog-owning households outnumber those owning cats, but that the number of pet cats is higher than dogs.Combined reptiles are the next popular followed by birds followed by horses.
source: http://en.wikipedia.org/wiki/Pet#Terminology