Griefing

Griefing refers to the act of a player ruining the gaming experience of another player, either involuntarily or on purpose (though this is debatable from one server to another - see below). The most obvious form of griefing is constant stalking of a player, by either following him or her for no reason, whispering useless messages, repeatedly sending them invitations after they decline, etc.

General Griefing - All these actions are against the ToS ("Terms of Service", as in TUA) and if reported action may be taken against the player or players involved.
 * Ninja looting (Though in groups or parties, it is a violation of ToS only if the raid leader explains the loot rules before the raid starts then goes against them.)
 * Spamming (Though all topics are usually allowed in General chat, unless they are profane or insulting.)
 * Spreading personal information about another player, even if it's false information.
 * Harassing, mocking, insulting players or gamemasters. (This includes creating pointless GM tickets such as asking for game tips or gold.)
 * Scamming, i.e. fooling others by pretending something that is actually not happening.

PvP Griefing - These actions, although some may see them as dishonorable, are considered legitimate PvP tactics and will not be addressed by gamemaster staff. Blizzard isn't against grieving behavior when there's a PvP remedy (if you can get together a group of allies to trounce the offending party or simply kill all the mobs quickly, the problem is solved), but when the griever is impossible to attack (is not set PvP, or is of the same faction as his targets), there is no such PvP remedy. Griefing like corpse and graveyard camping can be prevented on PvE realms simply by turning off your characters PvP flag.
 * Bluewalling
 * Corpse camping
 * Graveyard camping
 * Ganking
 * Kill stealing
 * Twinking
 * Training mobs