PvE  

ZAM Network Wikibase™ Dictionary

PvE is commonly thought of as the "normal" mode of play in most MMOs. Conflict is generally restricted to quests, monsters and other environmental obstacles. Players may not attack other players except in special circumstances (see Dueling and Arena).

This is as opposed to PvP, where players may or may not do quests, fight mobs and explore, but they may also attack other players.