Griefing is the deliberate, indiscriminate and repeated harassment of other players - with generally little actual benefit to one's own character.
It is essentially a "play style" where the player derives satisfaction from the discomfort he causes others instead from the accomplishments provided by the game itself.
This stands in contrast to unintentional bad behavior resulting from inexperience and also in contrast to harassment of specific players as can be part of a personal feud.
- Repeated killing of significantly less advanced characters
- Denying other players access to important resources by looting them aways immediately after they reappear.
- Denying less advanced players experience by out damaging them and claiming the kills for yourself
- Stealing the fruits of other player's labor by looting their slain monsters before they get around to it
- Convincing less experienced players to do things that will harm their progress.
- Training monsters to areas populated by player that cannot defend themself from them.
Like Mechko said: Bullying in the gaming world...