There's stats, splats, skills, rules, gear lists, special powers, spells, feats, merits, flaws, optional rules, scenarios, greybox text, NPCs, maps, art, "art", art?, tables, treasure, monsters, how-to lists, flowcharts, and indices. Some of it is good, some of it is bad. But it's not generally considered fluff.
Then there's fluff. The old World of Darkness splatbooks were famous for their fluff. Most were entirely fluff. For instance, I got Clanbook: Toreador (Revised) for two bucks. I bought it because I was running a chronicle with an NPC Toreador prince. 32 pages of 104 were non-fluff (4 pages of character sheet for Toreador, 4 pages of new Discipline powers, and 14 pages of sample characters and NPCs). The other 72 pages were fluff. Most of that was written "in character" and some of it was even written in fake handwriting font.
What is fluff? Well, to illustrate with CB:Toreador: The first 43 pages of the book are a history of the clan. This information is written from an "in character" stance for "flavor." The next chapter is a dissertation on what the Toreador do with themselves, including art, social snubbing, blah blah etc. Plus how they relate to other supernaturals. That's all fluff.
The next section is sample characters and NPCs. Though I liked the sample characters, I don't have much use for them. They were concise, with short "how to play this character" parts to mimimize fluff. The NPCs were awfully boring and almost totally useless. The NPCs were almost fluff, except they were at least intended to be directly useful out-of-the-box.