Player versus player (PvP) is a type of multiplayer interactive conflict within a game between two or more live participants. It differs from PvE which is Player versus environment. These are the two foundational modes of game-play which often drive how players design their character builds.
PvE builds are typically more versatile with multiple skills but not optimized for the most competitive combat between player factions. PvE characters are often trained in some Professions and usually don't need to use drugs. Players who start the game casually, usually register a Character build that is PvE by default.
Meanwhile, PvP builds are elite fighters often with high Luck and higher chance to do critical damage, with mostly combat perks. Each PvP Character is carefully designed for the most competitive combat. This includes Sneak characters if built properly. Players who do not carefully design a build will not usually end up with a proper PvP build, but a PvP build or a "broken build" at best.
Info Source: http://en.wikipedia.org/wiki/Player_versus_player