Most MMOs have at least some aspect of PvP (Player versus Player) interaction in them. In most roleplay MMOs, the main storyline and quests consist of killing non-player enemies (often referred to as mobs) and doing quests for NPCs (non-player characters). However, although most of the gameplay comes from killing computer generated enemies, there are also opportunities to fight other players. Sometimes characters can belong to rival factions, and in other games all characters are out for themselves and can choose to kill any other character that is getting in their way. Even in games where players are not encouraged to think of other players as rivals, the opportunity to enter battlegrounds or take part in duels allows for PvP. This is an extremely popular element of online gaming, as real people will often offer a better challenge than a computer generated enemy, and are usually considered a more satisfying kill.