When you kill another player in a PVP world, you either kill him justified (he had a skull when you killed him).
Or you had a skull and killed this player "unjustified".
And each unjustified kill you make, you will gain a "frag".
if you reach 4 frags, you will gain a red skull that will last for 14 days.
One frag last for 6 hours.
If you get up to 6 frags you will be banished for a week.
If you're playing on a pvp-enforced nothing happens, you'll only get a protection-zone lock for 15 minutes which means you cannot logout nor enter any places which are in protection zone (etc depot, temple and boats).