NPC is short for None-Playable-Character, which means creatures and characters that are in the game which are not actual players. When people talk about NPC they are usually referring to human or humanoid characters, while the rest are usually just called Monsters or Animals.
Mobs are living, moving game entities, which is another term for NPCs
These NPCs are friendly humans that can be interacted with. Since most all are innocent, hurting and killing them is a criminal act.
These NPCs are neutral when encountered, and most can be interacted with in certain ways. They only turn hostile when you attack them
These NPCs are hostile and will attack the player on sight. They can be calmed down, and some can even tamed.