A working dog refers to a canine working animal, i.e. a dog that is not merely a pet but learns and performs tasks to assist and/or entertain its human companions, or a breed of such origin.
Some might argue that the variety of often exclusive canine jobs is a better justification for the dog's honorary title "man's best friend" than the more accidental popularity as the most popular kind of pet in Western cultures.
More on [ Working dog ]

Best and Worst Dogs for Security - Ranks breeds for guarding, protection, and watchdog barking.
Meta Description: [ Ranks breeds for guard dog & protection duty. ]
Dog Detectives - An article from the New York Times discusses how dogs are trained to identify substances and people. Viewing requires free registration.
K9 Hero - Illustrated accounts acknowledge animals who have given more than expected of them, including those working with police, the military, search and rescue and as service dogs.
Meta Description: [ K9HERO.COM - We tell the story of the hero dog on the internet! We love dogs! ]
Working Dog Evaluation - A modification of the Puppy Aptitude Test used to predict temperament.
Working Dog Training - Free articles cover motivation, problem-solving and imprinting.