In short:
The VIS compiler does its best to make the rendered scene as efficient as possible when it's drawn in the engine. This means that it works out what it needs to draw and what it doesn't need to (i.e. something you can't see from a given position because it's obscured by a wall, barrel, pornographic scene, whatever). You use detail brushes to tell the VIS compiler what it doesn't need to worry about when working all this information out, usually because the brush you're applying the Detail to is just that - a detail, not part of the core of the maps visability. If your map is a sucession of rooms with 1 or more doors to the other rooms, most of the contents of the rooms want to be detail brushes as they are just details of the rooms, not part of the Room-Hallway-Room, etc... design.
Make sure to detail as best you can and you'll see a dramatic improvement in the performance of your compilers (and r_speeds in game, usually).
I myself have just been schooled on a couple of time-saving techniques of quickly detailing maps by Jits and a couple others, check out this thread for some of them:
here. (perhaps we can compile a little guide on DP Resource or something that describes these little time-saving tips?)
Looking forward to judging your submission!
Dag
