A build is a character's arrangement of statistics, classifications and scores for use with a particular set of roleplaying game mechanics. Can also sometimes refer to army composition in wargames. It is a term coined from a style of playing that emulates video game style character optimization, which is kind of meaningless when you can just make shit up and write it down anyway.
Munchkins and powergamers love to spend all their time on message boards talking about how effective particular builds are. Dungeons & Dragons 3e fans are particularly prone to this sort of rollplaying, as characters are highly customizable and there are lots of ways to break the game, especially since there are over 9000 supplements available to pick broken feats, spells, prestige classes and other goodies from. What every player forgets though is that no one on the internet cares how awesome your character is, and the DM will likely rocks fall, everyone dies anyway.