Extract from an article published by the Labrador Retriever Club USA:

“Within the Labrador Retriever breed type there are variations in body style which have evolved to suit the use of the dog, as well as the preferences of individual breeders and owners. In the United States the general public has begun to label these variations mistakenly as "English or "American" style. Perhaps a better description for variations in style is "show/conformation" or "working/field" styles.

The working/field or "American" style of dog is the label often attached to a Labrador possessing lighter bone structure and exhibiting more length of leg, a less dense coat, and a narrower head with more length of muzzle.

The conformation/show or "English" style Labrador is generally thought of as a stockier dog, heavier of bone and shorter on leg and with a denser coat, and having a head often described as "square or blocky." However, working/field variations occur in England as well, so this description is not necessarily suitable.”