Non Player Characters are units controlled by te game's AI that help players in many ways (See NPC Professions). They are mostly found in cities.
When a player interacts with an npc a window will open showing it's salute, and allowing to choose some dialog lines. In the lower right of the window, there's a menu with actions related to the npc's nature. Commerce, quests, premium, etc.
If the player talks to an npc as a part of a quest, the window will show an automatic dialog regarding the quest, instead of the original salute.