NPC stands for Non-Playable Character. There are NPCs that do things, from randomly engaging enemy mobs to giving you missions. The following are icons that appear above some of the important NPC types that you interact with in-game.

Supply Officers - Sells supplies for crafting

Medical Supply Officers - Sells medical supplies

Ammo Supply Officers - Sells ammo and grenades

Armor Supply Officers - Sells basic qaulity armor

NPC Mission Availability Indicator