Note: The article below belongs to Richard F. Stratton and is from his book "This is the American Pit Bull Terrier."
(Note that accepted breed names are treated as proper nouns are are therefore capitalized. Breed nicknames are treated as generic terms and are therefore not capitalized. It is difficult to discourage the use of nicknames especially when the formal name is as big a mouthful as our breed's - but my suggested treatment of nicknames will help us keep in mind that they are just that, nicknames.)
American (Pit) Bull Terrier.... The direct descendant of countless generations of game dogs. Although primarily famed as a fighter without equal, this breed has proved to be useful in predatory animal control and as a catch dog and guard dog - it just so happens that he makes an excellent house dog!
American Staffordshire Terrier.... The show counterpart of the APBT. Except for some game strains that are dual-registered , these dogs could not be expected to be as game as the APBT or to have to same ability.
Bull Terrier.... A show dog that was produced (around 1850) by a cross between the brachycephalic-type Bulldog and a terrier. Despite popular opinion to the contrary, this breed, because of insufficient agility and gameness, was never consistently used as a pit dog.
Bulldog.... The brachycephalic-type Bulldog now formally bears the name of his progenitor, the old original bulldog (now known as the American Pit Bull Terrier). Today the relationship to the APBT us extremely remote because the the breeds have been bred separately for so long. In fact, about the only thing the breeds have in common is that they are both dogs!
bulldog.... Nickname for the American (Pit) Bull Terrier. (It is important to note once again that the APBT almost certainly was the original breed that was known as the "bulldog.")
pit bull.... Another nickname for the APBT, one that has been in use for at least 200 years (probably to distinguish it from the brachycephalic-type "bull.") To further confuse the issue, the Bull Terrier people sometimes use this term to refer to their breed, apparently in the mistaken belief that their dog is, or was, used in the pit.
Staffordshire Terrier.... Former AKC name for the American Staffordshire Terrier
Staffordshire Bull Terrier.... An English show dog that is descended from many of the same ancestors as the APBT. However, it has been bred along different lines from the American Staffordshire Terrier and has very little similarity to the American (Pit) Bull Terrier.
Yankee Terrier.... A name that was concocted when the AKC refused to register that American (Pit) Bull Terrier under its own name. For some reason, this name, too, was rejected.