Griefing refers to a player intentionally disrupting others' enjoyment of a game. Exactly what actions constitute griefing will vary based on the setting, and it is up to each NWN server to determine exact guidelines.
While the guidelines do vary from server to server, some actions are commonly acknowledged as griefing. Foremost among these are hostile actions (e.g. attacking, picking pockets, laying traps) directed towards player characters, especially when these actions are repeatedly directed towards the characters of a single player or towards much lower-level characters, unless those actions are allowed under the server's PvP guidelines. (Even servers that allow some PvP may consider repeated hostilities to be griefing.) Other actions commonly acknowledged as griefing include (but are not limited to) taking the spoils of someone else's battle and obstructing the progress of other players. Generally, any attempt to interfere with another player's gaming could be considered griefing.
Griefing is prohibited on many persistent world servers.