Work

Dogs have lived and worked with humans in so many roles that they have earned the unique nickname, "man's best friend", a phrase which is used in other languages as well. They have been bred for herding livestock, hunting (e.g. pointers and hounds), rodent control, guarding, helping fishermen with nets, and pulling loads, in addition to their roles as companions. Service dogs such as guide dogs, utility dogs, assistance dogs, hearing dogs, and psychological therapy dogs provide assistance to individuals with physical or mental disabilities. Some dogs owned by epileptics have been shown to alert their handler when the handler shows signs of an impending seizure, sometimes well in advance of onset, allowing the owner to seek safety, medication, or medical care.

Info from: http://en.wikipedia.org/wiki/Dog