Any game where the player characters are acting against each other, rather than jointly tackling problems presented by a GM.
This is sometimes seen as a problem, as in many Traditional games, the intent is players being united against the GM's villains. This is compounded in many cases by the potential for hurt feelings and/or letting PVP in play bleed over outside the game.
PVP has often been embraced in rulesets as a viable way to play; in such cases, the GM is there to referee more than drive the plot.
In a subset of games embracing some PVP, specific rules for PVP actions exist. Exemplar games include Houses of the Blooded, Blood and Honor, Burning Empires and Burning Wheel.