From The Urban Dead Wiki
A catch-all term for intentionally annoying and disruptive behavior that is technically legal under the rules of a game. Being annoying isn't griefing in itself, but doing something simply to spoil other players' enjoyment is. Hindering other players' gameplay can be seen as griefing, depending on the context.
It has been argued that, for example, a survivor player who continually knocks down barricades or builds them so others can't enter buildings (see: EH-bitching), or a zombie who only attacks other zombies, might be considered a griefer. This, however, is a matter of perspective: all these actions can be seen as legitimate in-game tactics, playing styles and/or justifiable from a role-playing perspective. While such tactics as de- and over-barricading, generator killing, or ZKing may be irritating and inconvenient to those are affected by them, in and of themselves they do not constitute griefing.
But if an action hurts other players without benefiting the person in question, it could be considered griefing. This is especially true if the actions are repeatedly targeting or harassing a player or group. In particular, continually killing a specific player, over and over, is usually considered griefing. Killing zombies at Revive Points, if one has played the game long enough to be aware of them, is also seen by many (though not universally) as griefing.
Before it was changed, some players considered the skill headshot to be a de facto form of griefing because it removed XP from zombies without benefit for the shooter beyond the normal benefit for the kill (Damage Caused + 10 XP). See the headshot page for the history of the skill, including initial reasons for implementation.
Some don't believe there is such a thing as griefing because the actions can be explained from an in-character standpoint. More on this can be found on the Does griefing exist page.