There are hundreds of NPCs throughout Atlas! Some will hire you for quests, some will buy and sell goods - while others may teach you a special skill, or just provide you with helpful information.
Certain types of NPCs have floating icons over their heads, to help identify them:
*The Quest Giver icon only means that the NPC has a quest - however it may not be available to you currently (or at all, depending on the path you have chosen.) Likewise, this icon will not disappear once the NPC's quests have been completed - they only exist as identifying markers.