After killing another player, corpse camping is where that player then continues to camp at the enemy's corpse (either by using stealth or other means) and kills them when the player resurrects themselves. Although this is frustrating to the camped player, this is not considered griefing by Blizzard and is not subject to punishment by Game Masters.
Waiting for players to resurrect at a graveyard while flagged as PvP is also considered corpse camping, even thought there technically may be no corpse.
Corpse camping tactics
A person being camped will be cautious about resurrecting and will check around to see if you are gone or try to find some terrain feature that will prevent you from seeing where they are right away, allowing them time to escape. Rogues and Druids can simply Stealth/Prowl and become difficult to check for, and Hunters and Shamans may use Eagle Eye/Far Sight to monitor an area which they are camping from afar.
Don't forget to keep an eye out behind trees and buildings. For places where the terrain easily allows a person who is being corpse camped to hide immediately after resurrecting, spamming the following macro will help; immediately marking them with a Star if they resurrect and are within targetting range, allowing you to locate them easily:
/tar <person you are camping> /script SetRaidTarget("target",1)
Another tactic that can be useful once they know you are camping them is to make them think you left. Many times players refuse to resurrect until you leave. Log off. Wait a couple of seconds and log back in. When they saw you disappear, they may have assumed you joined an instance, BG, or simply logged off.
Corpse camping in Icecrown
Recently, the graveyard located at the Rise of Suffering near Icecrown Citadel has become a site for a 'new version' of corpse camping. When players die within the raid instance, they occasionally appear alive at this graveyard (instead of being in ghost form) with extremely low health. Players of the opposing faction who are patient enough can wait around until this occurs, after which they're able to easily one-shot the resurrected player. After this, the camper can corpse camp the killed player the 'normal' way, since the corpse of the victim remains at the graveyard. Camping in this fashion is considered extremely frustrating for the camped player, however, Blizzard hasn't made clear (yet) if these actions are considered griefing.