Before the days of dog shows and the purebred mating of champions, human recognised the merit in dogs that specialise in performing a specific job or task. We bred dogs that could hunt, herd, haul, guard, run, track and perform counless other tasks gearded towards making humans lives more comfortable, enjoyable, and manageable. Each dog's anatomy reflected the task that men set before him. The hunting dog had an insulated coat, a super sensitive nose, a short coupled body, a deep chest and straight, strong legs. The coursing dog had longer legs, a tucked-up abdomen (for speed), a deep chest (for lung capacity), keen eyesight, and a narrow, long muzzle (to slice the wind). The guard dogs were true heavyweights: massive and solidly boned with punishing jaws and nerves of steel.

The physical characteristics that set apart the staffordshire bull terrier are its impressive musculature, its strongly undershot strong jaws and large teeth, very pronounced cheek muscles, loose shoulders, roach black, low-slung body and long legs that bend in the forequarters. These are the characteristics of a fighting or baiting dog that enable it to perform the tasks that breed indeed all the bull and terrier dogs, were created to tackle. The decree'Go Low, pin and hold!' was in sooth a battle cry! This imposing physique was needed for the dogs to fight one another, as well as dodge and grab an ornery bull with their powerfull gripping jaws and hold on to it without being tossed aside!

Baiting a bull, an animal twenty or more times the size of a dog, placed some obvious demands on the dog, its anatomy and temperament. The desired temperament of a bull and terrier dog for baiting was not a vicious, risk-taking daredevil. Instead, the baiting dog required an even-keeled, leve-headed, obediend temperament, peppered with patience, indomitable courage and tenacity. The bulldog excelled in the pinning and holding of the bull, but lacked the flexibility required in the dog pit. thus, the smaller bull and terrier dogs were designed to take on this challenge and each other.

Official Standard of the Staffordshire Bull Terrier
General Appearance: The Staffordshire Bull Terrier is a smooth-coated dog. It should be of great strength for its size and, although muscular, should be active and agile.
Size, Proportion, Substance: Height at shoulder - 36 to 40 cm. Weight - Dogs, 12 to 17 kg; bitches, 10 to 15 kg, these heights being related to weights. Non-conformity with these limits is a fault. In proportion, the length of back, from withers to tail set, is equal to the distance from withers to ground.
Head: Short, deep through, broad skull, very pronounced cheek muscles, distinct stop, short foreface, black nose. Pink (Dudley) nose to be considered a serious fault. Eyes - Dark preferable, but may bear some relation to coat color. Round, of medium size, and set to look straight ahead. Light eyes or pink eye rims to be considered a fault, except that where the coat surrounding the eye is white the eye rim may be pink. Ears - Rose or half-pricked and not large. Full drop or full prick to be considered a serious fault. Mouth - A bite in which the outer side of the lower incisors touches the inner side of the upper incisors. The lips should be tight and clean. The badly undershot or overshot bite is a serious fault.
Neck, Topline, Body: The neck is muscular, rather short, clean in outline and gradually widening toward the shoulders. The body is close coupled, with a level topline, wide front, deep brisket and well sprung ribs being rather light in the loins. The tail is undocked, of medium length, low set, tapering to a point and carried rather low. It should not curl much and may be likened to an old-fashioned pump handle. A tail that is too long or badly curled is a fault.
Forequarters: Legs straight and well boned, set rather far apart, without looseness at the shoulders and showing no weakness at the pasterns, from which point the feet turn out a little. Dewclaws on the forelegs may be removed. The feet should be well padded, strong and of medium size.
Hindquarters: The hindquarters should be well muscled, hocks let down with stifles well bent. Legs should be parallel when viewed from behind. Dewclaws, if any, on the hind legs are generally removed. Feet as in front.
Coat: Smooth, short and close to the skin, not to be trimmed or de-whiskered.
Color: Red, fawn, white, black or blue, or any of these colors with white. Any shade of brindle or any shade of brindle with white. Black-and-tan or liver color to be disqualified.
Gait: Free, powerful and agile with economy of effort. Legs moving parallel when viewed from front or rear. Discernible drive from hind legs.
Temperament: From the past history of the Staffordshire Bull Terrier, the modern dog draws its character of indomitable courage, high intelligence, and tenacity. This, coupled with its affection for its friends, and children in particular, its off-duty quietness and trustworthy stability, makes it a foremost all-purpose dog.
Disqualification: Black-and-tan or liver color.

Sources:

History-
http://www.staffy-bull-terrier.com/history

Breed Standard-
https://www.akc.org/breeds/staffordshire_bull_terrier/breed_standard.cfm