Player versus Player (PvP) is a type of "personal battle" against another player, or a group of players. Traditional PvP is similar to duels, but are not based on killing the other participant(s).
There is a kind of PvP called Player Killing (PKing), which results in the death of one of the participants. As a result, the killer is given a red cursor to signify such crime.
The player may naturally do it on purpose as a source of income and loot. This will make you a PKer (Player Killer), and will make you a target for Bounty Hunters, Anti-PKers and Anti-PKing guilds.
There's also the possibility to duel with other players by sending them a duel request. Duels will be based on a "first hit" system, and a mode where you need to lower your opponents health down to a certain amount. There will also be timed duels where whoever has the highest health when the timer runs out will be the victor.