Herding - Herding breeds were originally bred to help control livestock, such as sheep and cattle. They tend to be agile and intelligent, easy to train, and responsive to human commands. This is why some herding breeds, such as the German shepherd, also make excellent police, military, and search and rescue dogs.
Working - Working breeds are those bred to perform specific jobs that don't involve hunting or herding. This includes sled dogs like the Siberian husky, search and rescue dogs like the St. Bernard, and larger breeds like the Rottweiler, which the Rottweiler Club of the United Kingdom says was bred to guard cattle being driven to market.
Non-sporting - This group is for breeds that aren't easily categorized into the other groups. Non-sporting dogs include the Dalmatian, the Poodle, and the Chow Chow, as well as other dogs bred simply for companionship or to perform roles that don't fall under the other main categories.
Toy - The toy group includes all of the tiniest breeds. Some, such as the Yorkshire Terrier (terrier group) or the Toy Poodle (non-sporting group) would be classified elsewhere if not for their diminutive size. Generally, under 10 pounds, these are lapdogs bred primarily for companionship.