Some quests will require the collection of a certain object. Normally these are special items that will only appear to someone who has the appropriate quest and does not have as many as they need. These items, which are marked as "quest items," cannot be traded or sold, meaning that each player has to collect them themselves, and that earning them in a group is usually somewhat fair.
If a group is set to Group Loot or Round Robin, and the person who should loot a corpse does not need the quest items on it, the corpse will be open to anyone who can take the item, but they will only be able to loot the quest item. (This also applies to other potentially unlootable items, such as Scourgestones made available by equipping the Argent Dawn Commission.)
Quest items for a given quest will be removed from inventory if the quest is abandoned.