I know that in WorldCraft (a program for creating levels in games such as half-life) a pre-fab is a prebuilt object which can be placed in the level (like a table or a chair). I'm not sure what it is in 3D Studio MAX or AutoCAD but I'm assuming its something silimar.
If that is what you're looking for, then check out Ed Sparks FIRST CAD Library at
http://sharingfirst.mit.edu/users/edsparks/.