Just like item farming, gold farming is the process of going into high-level areas and repeatedly killing the same monsters to get the best items and/or most gold, then selling it for actual cash. Wiki's got a better description than me.http://en.wikipedia.org/wiki/Gold_farming
Gold farming is more WoW, which I don't play, but item farming is everywhere, even Guild Wars... Which I do play. Personally, I'm not a fan of the concept. It tends to upset game economies.