Player versus environment, (or PvE, sometimes player versus monster, or PvM), is a gameplay distinction, in contrast with player versus player (short: PvP) gameplay.
Player versus Environment is a type of gameplay common to RPGs in general. This is where the character engages in combat with AI-controlled computerized opponents, as opposed to PvP, where she/he engages with human players as opponents.
In Guild Wars, PvE content is available to roleplaying characters. However, a PvP character can engage a set of computer controlled opponents known as the Zaishen (especially during tutorials) as well as other players. In PvE gameplay, the primary purpose is to complete the storyline. In addition, PvE players can "grow" their characters, gain experience, learn skills, acquire better weapons and armor, improve titles etc. This is done mainly by completing quests and missions, venturing into explorable areas to fight monsters, and farming.