A Quest is a task that the player can complete for a reward. Usually, quests are prompted by NPCs; however, this is not always the case and quests can also be prompted by travelling into a new area or reading a tome.
If prompted by an NPC, he or she will have an "!" above his or her head and move towards the player to indicate that they have a quest. Clicking on the NPC will open a dialogue in which players can see what he or she wants.
The quest log (accessed by pressing Q) assists players in quests by reminding them of objectives and appearing when players are near an objective concerning the quest. The quest log also saves dialogue from NPCs that give out useful information regarding the missions.
Completing quests usually yields rewards. This can range from additional Skill Points, Stat Points, an item, or access to later parts of the act. The last quest of an Act always grants access to the next Act or Difficulty.