PK stands for Player-Killer and means people who try to kill other player characters in games that allow killing other PCs but promote cooperative adventuring: Everquest (aka Evercrack), Ultima Online, MUDs, the aforementioned Diablo games, etcetera.

In Everquest one has to choose to be PvP killable by other PCs by giving a book to the Priest of Discord, but in most other games it is possible to be killed by surprise, one way or another (sometimes by a hack).

Someone tried to do it to me in Diablo II the other day. I don't really play these kind of games that often, so this was my first experience with a really obnoxious type of PK.

I was on the European server of Diablo II's Battle.Net and a Barbarian asked me to group with him. I accepted the invitation, and asked him where he'd like to go questing. He suggested the Arcane Sanctuary, a very cool level in which the characters appear to be walking along hallways suspended in outer space.

I arrived and waited a minute or so for him to get there. No sooner had he traversed the portal to the Arcane Sanctuary than he went hostile with me and attempted to attack me. He was a couple levels higher than I was, and I immediately used the portal to return to town.

He proceeded to call me a "woman" (I found this funny) and coward for not fighting him. I reported him but I think nothing transpired of this. I still don't know if what he did was just rude or actually against rules.