There are 5 types of character PvP modes.
Peace: only attack monsters.
Friend: attack monsters, oponents with red/grey name.
Team: attack monsters and opponents.
Guild: attack monsters and players except guild members.
All: attack monsters and all players.
Tips: if you kill too many other players, your PK points will increase. Once your PK points exceeds, your attack and defense will be decreased as a punishment. Staying Online and/or using an Atonement to reduce your PK points.