Fluff

Fluff is gamer slang for the stories, lore, and in-character description of a system or setting; it is the opposite of crunch, which solely describes mechanical effects. For example, in Games Workshop's official fluff for Warhammer 40,000, the Space Marines are extremely hardcore superwarriors who regularly achieve kill-to-death ratios somewhere in the order of several thousand to one; crunch-wise, although tough, their statistics are not nearly as good as the fluff would suggest.

Powergamers typically ignore fluff to focus on crunch; however, the opposite extreme, a player who designs characters based solely on the fluff associated with their choices and ignores the crunch, can be just as annoying, as they have a tendency to produce characters that are very interesting and well fleshed out, but nevertheless cannot actually do anything useful within the game mechanics.

Game mechanics such as Fate or Dogs in the Vineyard strive to make character background and personality itself a mechanical advantage/disadvantage, allowing for well fleshed out characters that are mechanically effective at the same time.

In games such as Warhammer 40k, some people take their fluff seriously. If it isn't grimdark enough or noblebright enough or manly enough or weaboo enough or subtle enough they throw bitchfits. You can often find them scurrying up and down the site, gnawing on the ankles of articles desperately trying to find an outlet for their rage. Skub ensues.

Fluff can more often than not make or break a faction or game. An otherwise questionable game can rake in a huge fanbase if the fluff is appealing, while an otherwise solid, efficient army force becomes despised by its horrendous, Mary Stew riddled fluff. Insert your own Matt Ward reference here, it hurts too much.