Two usages for "mod" exist in the language of computer gaming:
a) hardware modification to make a PC look better (and often also run silently)
b) software modification where hobbyists change aspects of a game. E.g. a group of fans modified "Battlefield: 1942" to use modern weaponry; in order to play this, one needs to install the "mod" over the game folder.
Der zweite Wortsinn fehlt in der von mir oben vorgeschlagenen Begriffspaarung.
"Bastler" is similar, but I'd label anyone who assembles PCs or toy airplanes a "Bastler". A "Modder", basically, does the same thing, only they would try to get his PC to look like a toy airplane.
US modder = someone who modifies a PC to look better
The word in use among German modders:http://www.dexgo.com/index.php?site=stuff/was...
Another fan site:http://www.modding-station.net/index.php?chap...
Presumed Etymology: "to mod" = "to modify"