PvE stands for Player vs. Environment and is a play style where players play in and against the game world environment, such as crafting, questing, running instances like raids, dungeons, adventures, or shiphands, world events, or just roaming about the world freely. The core reason for the term is to differentiate regular game world play from PvP, or Player vs. Player play.
In Realms Edit
By default most realms are designated as PvE or regular rule server. Some realms are marked as PvP and and have special rules for how players can engage each other for fighting.