Hmm - I see the definition of "fluff" is become rather broad.
I know that when I see "fluff", I think of the descriptive writing that goes into supplements. Things like "The trees get golden leaves in the winter". Stuff that only really serves a purpose as a possible inspiration for the DM and players.
I see fluff as something complete different to good old-fashioned
advice, something that you see quite a lot of in the DMG (thanks, Monte!) and somewhat in other books. (In the
Complete Warrior, for instance, there is advice on running a Mercenary/War campaign). Dungeoncraft is an excellent advice column. (Though when Ray Winninger was writing it, the fluff content got quite high towards the end, as his world didn't interest me at all!)
There are also
Adventure Seeds, which are more focused than general descriptive writing. "The Zombie Master is missing, lost in a nearby tar pit." These can be inspire single encounters to entire campaigns. (I rather like the random encounter tables in Midkemia Press'
Cities supplements.)
I tried today to read through
Dungeon Magazine's description of Hardby... and just couldn't make it. It just wasn't relevant to me. I did feel I was drowning in irrelevant details.
You can probably categorise further elements of non-rules writing. Where, for instance, does the Magical Medieval Society fall?
The divide is not simply Fluff vs. Crunch, and it is important to keep that in mind.
Cheers!