Every game has its own import/export set of tools, For instance I believe UT2004 only accepts UnrealEd-made models, or those following the same format. Halo CE on the other hand (PS. CE's the better version of PC free for anyone with a valid version of PC, custom maps allowed) uses a program called Blitzkreig which takes models from 3ds Max 5/6 and imports them into a JMS file format, which then is mixed in to a .map file with all the bitmaps, sounds, triggers, music, etc.

There's no defined convention... just go and look at modding sites for whichever game you wish to mod, the tools should be there. Note: Most games prefer 3ds max over Maya. Some use GMax, which is a cut-down version of 3ds max for free.

The basics behind level making, however, is that you have a giant 3d model and people walk on it.