The breed standard considers two types, the Scott (also called "Performance" or "Standard") type, and the Johnson (also called "Classic" type.) These are named for the two men who each bred their dogs for certain distinct qualities

The Standard / Performance / Working / SCOTT Type:

The Scott type looks like a large, coarse, leggy, white Pit Bull. They are lighter weight than the Johnson type and have a very muscular athelic look and their front legs are under their body, not thrown out to the sides.

The Bully / Classic / JOHNSON Type:

The Johnson type resembles an athletic, tightly built, white Bullmastiff. As compared to the Scott type it is a larger, wider dog with more bone, pendulous lips, and undershot jaw, facial wrinkles and a shorter muzzle.

However, many modern American Bulldogs are a combination of the two types

The "Hybrid" Type:

These are basically bulldogs whose blood-lines are a mixture of standard and bully types, with the goal being to distill the best features of both. Today most bulldogs are probably in this category.

General Appearance:

The American Bulldog is a well-balanced, short-coated, muscular and athletic animal. American Bulldogs display great strength, endurance, and agility. Males are characteristically larger, heavier boned and more masculine than the females.

Temperament:

American Bulldogs should be alert, outgoing and confident. Aloofness with strangers is acceptable.

Some assertiveness toward other dogs is not considered a fault.

Faults:

Overly aggressive or excessively shy.

Size:

Standard Type:

Ideal standard males should measure between 23 to 27 inches at the withers and weigh from 75 to 95 pounds. Females; 21 to 25 inches, 60 to 85 pounds.

Bully Type:

Ideal bully males should measure between 23 to 27 inches at the withers and weigh 90 to 115 pounds. Females; 22-26 inches 80 to 105 pounds.

Note:

The overall proportion of the dog is of utmost importance when evaluating weight.

Head:

The head should be broad, flat on top squared appearance with a well defined stop. It should also be medium in length with pronounced muscular cheeks.

Standard Type:

A box or wedge shape is preferred.

Bully Type: A larger rounder shape is ideal.

Muzzle:

The muzzle should be broad with wide-open nostrils. The muzzle should be wider at the base and taper to the nose. The lips should be full with black pigmentation; some pink allowed. The chin is well defined and must not overlap the upper lip nor covered it.

Standard Type:

Muzzle should be medium in length 2 to 4 inches. It should also be 35% to 45% of the overall length of the head.

Bully Type:

Muzzle should be broad 2 to 3 inches in length and should be 25% to 35% of the overall length of head.

This is just a general overview of the difference in types.