Quest items are items which are either only obtainable while a quest is active, or are solely useful inside a quest (i.e. have no useful function when their related quest(s) are inactive). They are not usually possible to trade to other players, but there are some exceptions. Notably, the broken shields must be traded with another player in order to complete Shield of Arrav.
In situations where it is not normally possible to obtain multiple copies of the same quest item, the drop trick can usually be used.
For a list of quest items, see Category:Quest items.