Mapping or level designing is the term of making virtual worlds for computerbased games ;P .. Actually the process is pretty hard, it takes some medium skill and TALENT.the most important factor of being a virtualworlds maker..
I've been into this for 2 years now(almost) ,and have made some kewl levels for online multiplayer games as Actionquake2, Counterstrike and now i'am into
Urban terror for quake3. (all these games are very violent)
The best way to make levels or worlds is having an image of a cool place in your head , or on paper,. and then you make it real by using the Editor.
My dream is being a professional level designer , but since
the demand is so small , (eg working for game industry)
AND there's practically no education aiming at gamedesign here in scandinavia, i'am pretty unkewl about that.. well sheit
(i would prefer the Electronic Arts GameDesign Institute in Houston U.S) :D
The editors i'am using are basically Worldcraft and q3radiant,
check out their respective pages ^ for more information

Beneath is some of the crazy stuff i've mapped during this year