Player versus Player (or PvP), is a gameplay distinction, in contrast with Player versus Environment or PvE gameplay.
Player versus Player is a type of gameplay. This is where the character engages in combat with with human players as opponents.
In PvP gameplay, one can team up with up to 5 players to create a Party. Your primary purpose is to fight your way through a Dungeon to obtain special types of Armor, Weapons and Items. Your Party may encounter and fight another group of human controlled players.