A simple rundown:

1) You choose to be a playerkiller if you desire so.
2) You can make this choice at level 20+.
3) If you ever die in a pk fight, you are made unpk.
4) You may choose to go pk as often as you like.
5) The death penalty for pkdeaths is your level, and all stats greater than your level + 3.
6) You get a lot of experience for killing a player.
7) Type 'help pkrange' to read about who you can/cannot attack.
8) You may get set PK if the Wheel hits you.

See also: playerkiller, pklist and pkills.