Help - reading in data

I've read a ton of articles covering practically ever subject involving programming 3d graphics. What I haven't seen, and what my question pertains to is
reading data into your graphics/game engine.

Lets say, I would like to program a Quake-style FPS. What is the theory behind reading in all the vertices, texture maps, etc. that make up a level. Do you
read in a whole level's worth of data at once? Do you continually read data prior to each transformation adn redering?

Your help is greatly appreciated.