Often, the objective of the quest involving the quest item is simply to deliver the item to another NPC in a different location (such as delivering a message). In another case, you may need to explore an area to find a certain quest item, and then return it to the NPC who gave you the quest for a reward. If you come across a quest item while exploring and don't have a quest to go with it, it's a good idea to hold on to the quest item; you should find the quest involving it later on.
Quest items cannot be salvaged or used in any way (with the exception of the Kilnn Testibrie quest items from the Great Northern Wall mission, which can be sold to merchants and salvaged). They will take up a slot in your inventory until you complete the quest, or drop or destroy them. Most cannot be sold to merchants or traders due to their zero gold value, but they may be sold or traded to other players. Beware of the Althea's Ashes quest item scam.
To see a list of quest items see Category:Quest items.
Collectable quest reward items
There are some collectable quest items that are given as rewards for completing certain quests. These items are stackable and can be traded by respective NPCs for useful items like identification and salvage kits, scrolls or even gemstones. Because of this these quest items are fairly valuable and often traded between players.