A quest is a task given to a player character that yields a reward when completed.
There are three types of icons. '?' = This npc has a new quest, '!' = You have an active quest, 'Q' = You have a completed quest
When you right click a NPC which has a quest, the quest gui opens. Each wool represents a quests, wool color changes according to quest state.
You can see your current quests and their status on tablist.