NPC is the acronym commonly used in games for Non Playing Character or Non-Player Character. There are 3 main types of NPCs. They are:
Quest NPCs are NPC's that must be spoken with to start, continue, or finish a quest. If they are involved in a quest you have not started, but are able to, then the NPC will have a lightbulb over their head. If they are involved in a quest you have started, then they will have an open book above their head. If you must speak with them to continue a quest you have started, then they will have a closed book over their head.
Not to be confused with Storage NPCs. Certain NPCs have stores that enable them to buy from and sell to players. It is worth noting that some stores will sell items cheaper than others, depending on their location. Every town has some type of store-running NPC.
Not to be confused with Store NPCs. These NPCs are used to access your storage area. Anyone can store items, but there is a space limit. This limit can be increased by 4 slots for 4,000 Nexon Cash in the Cash Shop. This limit cannot be expanded above 48 slots.