From Diablo Wiki
PvP = Player versus Player combat. Characters fighting other characters, instead of/in addition to fighting the monsters.
Types of PvP
There are various types of PvP in Diablo II, and other RPGs.
- PKing is Player Killing, non-consensual attacks, usually by a character who specializes in PvP against unsuspecting PvM characters.
- Dueling is PvP combat between consenting characters. It's usually held in a specially-created dueling games, between characters who are optimized for that type of combat.