The DM's greatest tools are his authority over the gameworld and his ability to limit/expand the players' knowledge of it. Etch that on a stone terminus and refer to it when you get the desire to say too much.
I think it's natural for us to want to create borders around our imaginary worlds. Doing so increases their comprehensibility, both to ourselves and the players. Like them, WE want to understand what's going on in this strange place.
But consider the possibility that too much of this comprehensibility stuff may be a bad thing.
The best sort of player is a worried player. Not to the extent that she's ready to commit suicide if Black Leaf should bite it, of course. But nervous enough to put her whole mind into the game. If she feels dread, if she's concerned with the fate of her character, then she is immersed and you've done your job.
Instead of elaborating on the game world's minutia and potentially burying the players in background, IMHO the DM should be concerning himself with how to create the right level of tension. This certainly isn't going to happen by telling the players too much [increasing comprehensibility] about their environs in a second-hand way. If anything, such details should be difficult to fully obtain -- and slippery [potentially inaccurate or perhaps only partly true]. Travelers exaggerate. Old wives lie. Peddlers bend the truth as it suits them. The players should not know whether or not the world truly be round or flat, whether the sun be merely a disk or a giant orb of exploding gas. In order to lay the groundwork for dread, this sort of concrete knowledge should be fragmented and obscured before it reaches the player. Keep them guessing. Ram home the idea that first-hand experience is the only thing they can fully rely on.
I admire James M's approach to this stuff in his Dwimmermount campaign. Raw information about the world is revealed through gameplay. Stumbling across cultural artifacts and analyzing them as a byproduct of the dungeoncrawl allows the DM to toy with player knowledge. The goal here should be to create a healthy level of paranoia and skepticism. These things will go much farther to increase the believability of your world than all those reams of notes gathering dust in your filing cabinet.