CS mapping i abit like modeling, but you got a grid you have to follow. You cant make an object that is 1/2 square big unless you make some sorrt of triangle. You start off by making a box, then you cut it as you wish, but you must cut right through.But it's not purely terrain (i think). Think there are spawn locations, lights and such you must place in or you can't see or do anything whilst playing. I tried to make a CS map once, i personally think the entire program is a mess. Not very smart to list 1000 different textures.
To sum it all up, CS is like 90% esthetical.