USA refers to cars sold in the U.S or Canada market. USA cars are easy to identify, as they are on full-sized cards, not on the shorter (INT) cards, and are written in english. USA cars will sometimes have different Toy numbers, and collector numbers, but usually contain the same color and tampos as (INT) cars.
USA cars sometimes receive exclusive colors, such as in some Superfast series. They can be found just about anywhere in the USA or Canada.