The Staffordshire Bull Terrier (informally: Staffie, Stafford, Staffy or Staff) is a medium-sized, short-coated, old-time breed

Dog breed

Dog breeds are groups of closely related and visibly similar domestic dogs, which are all of the subspecies Canis lupus familiaris, having characteristic traits that are selected and maintained by humans, bred from a known foundation stock....

 of dog

Dog

The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

. It is an English dog, where it is the 5th most popular breed, and related to the bull terrier

Bull Terrier

The Bull Terrier or English Bull Terrier is a breed of dog in the terrier family. They are known for their large, egg-shaped head, small triangular eyes, and "jaunty gait." Their temperament has been described as generally fun-loving, active and clownish...

. Having descended from dog-fighting ancestors, it is muscular and courageous, but the modern breed is now known for its stability and affinity for people - despite this it is the subject of breed specific legislation in some jurisdictions.
The Staffordshire Bull Terrier is a medium-sized, stocky, and very muscular dog with strong athletic ability, with a similar appearance to the American Staffordshire terrier

American Staffordshire Terrier

The American Staffordshire terrier is a medium-sized, short-coated American dog breed. In the early part of the twentieth century the breed gained social stature and was accepted by the American Kennel Club as the American Staffordshire Terrier in 1936...

 and American pit bull terrier

American Pit Bull Terrier

The American Pit Bull Terrier is a medium-sized, solidly built, short haired dog whose early ancestors came from England and Ireland...

s sharing the same ancestor.
They have a broad head, defined occipital

Occipital

The word occipital, in zoology, pertains to the occiput .Occipital is a descriptor for several areas of animal & human anatomy.*External occipital protuberance* Internal occipital crest* Greater occipital nerve...

 muscles, a relatively short foreface, dark round eyes and a wide mouth with a clean scissor-like bite (the top incisors slightly overlap the bottom incisors).