PvP (Player vs Player) mode lets you attack and fight with other players in the game. PvP is only available once you reach LV50, and depends on the color of the current zone. Green Zones are safe areas, where PvP is permitted. White Zones are Neutral Zones, where PvP is available but attacking non-Red players is penalized. All other zones (Red, Orange, and Blue) are Arena Zones where PvP is available.
Even if you have PvP disabled, you may still be attacked outside of Safe Zones. PvP settings only determine if you can attack other players. Players with PvP enabled will be marked with a bloody sword icon next to their name.
Click the PvP icon below your character status to open the PvP settings window. Use the radio buttons to switch PvP mode on and off, and the checkboxes to decide which players you want to be able to target. You can also quickly enable or disable PvP by pressing CTRL+A. You can disable targeting on members of the same faction or clan as you, people in your party or your friends list, and Red and White name players. White name players are players with 0 infamy, while Red name players are players with more than 0 infamy (PKers). Attacking a Red name player has no penalty, while attacking a White name player will have a penalty in White zones, but not in red, blue, or orange zones.