American pale ales are distinguished from British pale ales mainly by hop strength and hop variety. British pale ale, sometimes called “bitter”, generally has a bit less hop bitterness, flavor, and aroma, and traditional UK hop varieties tend to be softer and more floral than those in the U.S. American pale ales are more aggressively hopped, and U.S.-grown hops tend to have a bit more citrus character.
-From 365 Bottles of Beer for the Year
