Quest items are a type of item used in missions and quests to fulfill some objective. They are white and have the words "Quest item" in their description. They occupy one slot in the inventory and cannot be salvaged or used in any other way. They can be destroyed using the bin in the character's inventory. They are worth 0 gold so they cannot be sold to NPC traders, though they can be traded to other players.
Most quest items are obtainable when the associated quest is active. They may be given out by the quest giver when the quest is accepted, dropped from an enemy that is killed, or retrieved from a specific NPC or interactive object. Occasionally, you can get quest items even if you don't have the associated quest. Holding on to these items might prove wise, either to sell to others or to use it for solving the associated quest.