FTW is an acronym for "for the win." It has at least two possible sources. It likely stems from sports announcers in sporting events, especially soccer in Europe, that will end a match by saying "<winning team> for the win!" It is also a gambling term in at least the United States, where you would bet on a team or race by stating "<competitor> for the win."
In online games, it is meant to express general approval for a group or idea. For example, an overzealous protection monk, pleased with his build, might say "Protection Monks FTW" when defending his choice in chat. It is often used instead of a generic owned.
A similar expression, used to express general disapproval, is FTL - "for the lose."