What is Player vs Player?
In MMORPG's Player vs Player (PvP) is a style of play that allows players' characters to adversely affect the characters of other players, primarily through combat. World of Warcraft is a well known example. Balanced PvP has been a concern for players and game developers alike for decades. In the early days of PvP, higher level players would "grief kill" (kill another player's character just to cause grief for the victim). This not only adversely affected the victim, but the community (and game company) as well. As such, many MMORPG's restrict PvP interaction in a variety of ways.
The safe haven method creates an area in the game world wherein PvP in strictly controlled. For example, no PvP activity can be conducted within a certain range of a city. In some games, opposing faction characters are unable to even enter these safe havens. This allows players that don't want to engage in PvP an area where they can avoid it. Most safe havens are located near areas where new characters are made, thereby allowing new players a chance to learn the game before being too seriously endangered.
In this method, PvP actions can only be performed on a player character which level is within a certain range of the perpetrator. An enemy character might be able to be in the vicinity, but will be unable to attack (or be attacked by) another character too high or too low.
Some games offer servers with full PvP capability. Players that desire PvP can play on these servers with other like minded players. If a player wishes to avoid PvP, he merely needs to play on a non-PvP server.