A build is a collection of 8 skills, an attribute point distribution and equipment to be used. The term "build" is synonymously used for single player builds and team builds consisting of multiple single player builds meant to be used together.
Since level differences are absent in high end PvE and almost all forms of PvP, builds play an important role in Guild Wars. The outcome of any battle is decided by player skill and the builds used (not to mention a bit of luck). Builds for various tasks exist, the most important of which are prominent enough to be recognized by name by many players in game. A build will usually be named after the elite skill that it uses, a specific skill combo that it takes advantage of or what the build was meant to farm in PvE. Sometimes it will be named after an unusual characteristic that makes it unique.
Builds are often created and discussed on various fan pages. While successful PvE builds spread more slowly and stay around for a long time, successful builds in the PvP area of the game can spread quickly and lead to a Flavor of the Month syndrome, where a huge proportion of builds run in one particular setting are identical. Such dominance by one family of builds often leads to nerfs by ArenaNet.