Definitions vary on what a working dog is, they are sometimes described as any dog trained for and employed in meaningful work; other times as any dog whose breed heritage or physical characteristics lend itself to working irrespective of an individual animal's training or employment; and other times again it is used as a synonym for herding dog.   
Roles performed by dogs that sometimes sees them classified as working dogs include:
Working dogs are known as an Australian icon. They are prevalent in Australia due to the large agricultural industry. Working dogs can be seen herding cattle or sheep and are also used to guard poultry from foxes and cats. A report in 2019 stated that there were over 270,000 stock herding dogs in Australia.  Some popular breeds for farm work include Border Collies, Kelpies, Maremmas and Australian Cattle Dogs.
Events to showcase the skills of working dogs are held through Australia alongside key local festivals and events. Trials and performances include high jumps, sheepdog speed trials and sled or weighted pulls. 
When competing in conformation shows, a number of kennel clubs classify various pedigree dog breeds into a "working group" or "working dogs group", although it varies between kennel club what breeds are so classified. The Kennel Club classifies mastiff, pinscher, sled dog, select livestock guardian dog and some other breeds in their working group.  The American Kennel Club's working group and the Canadian Kennel Club's working dogs group are very similar to The Kennel Club's except they include all livestock guardian dog breeds and all full sized spitz breeds (not just sled dogs) recognised by the clubs.   Both the Australian National Kennel Council's and New Zealand Kennel Club's working dogs groups comprise herding dog breeds and select livestock guardian dog breeds recognised by those clubs.   The Fédération cynologique internationale has no working group. 
a dog of suitable breed or training kept for its practical use, such as herding sheep, rather than as a pet or for showing
a dog suitable by size, breeding, or training for useful work (such as draft or herding) especially as distinguished from one suitable primarily for pet, show, or sporting use
A dog trained to work with livestock.
any of various breeds of dog bred as guard, herding, draft, or rescue animals.