In Diablo II, a player shoots a missile at another player while outside of town. They are either partied or unhostile to eachother. The common examples of missiles are Ice Blast, Tornado, Guided Arrow and Fire Bolt. The player who shoots has gear that maximizes their damage, usually minimizing their survivability. This makes pkers fragile and is why most don't duel. Anyways... after the missile is fired, a hack takes them to town and hostiles their target instantly in literally 0 seconds. The missile fired now hits the target and usually kills them. Pkers can be any character, but are usually high level sorceresses or low levels killing other low levels in Enchant games (a sorceress Enchanting low levels so they can level faster). Usually the pker just goes up to a person and tppks, hosts Baal runs and pks, or camps a waypoint/townportal and pks.
Since on hardcore mode, a player can only die once, this invites a legion of people who create characters for the sole purpose of ruining someones gaming experience. It also adds a sense of excitement and an atmosphere of distrust to hardcore mode. No one Baals in Hell on USEast realm, for example, unless it is a private and safe run.
Sometimes they have a script that automatically types out a sometimes clever message or series of messages (sometimes to form a picture or large word) that follows 0-1 seconds after the pk.
---An example of a tppk in a normal difficulty Baal run on hardcore:
LowLevelNoob: thankz 4 hostin baalz i can almost were p crown!!1
MyPkerSorc: No problem.
LowLevelNoob: most runs i join sum nuob trys to pk me lol
MyPkerSorc: I hate pkers.
LowLevelNoob: me 2
MyPkerSorc: What is your name?
MyPkerSorc has expressed hostility towards you.
MyPkerSorc has left your party.
LowLevelNoob was slain by MyPkerSorc
MyPkerSorc: LowLevelNoob was owned.
MyPkerSorc: Back to level one! LOL
LowLevelNoob (*IHatePkers) has left our world. Diablo's minions weaken.