If you play word-association and use the word “fluff” on someone from the UK, they will probably say “belly button”. If you tried it on an American, however, they would likely say “Marshmallow”. It's not because they're insane, it's just that popular there.
Great for baking, a sweet sandwich spread (try it with peanut butter; that's called a fluffernutter apparently), or smeared on the face as part of an emergency Father Christmas costume, it has a spread consistency and it tastes of marshmallow. The city of Somerville, Massachusetts has an annual festival to celebrate the invention of it. The “What The Fluff?” festival is normally in September. Seriously – there are hats.