An NPC, or Non Playable Character, is a fictional character not controlled by a player. In electronic games, this usually means a character controlled by the computer. In traditional tabletop role-playing games, the term applies to characters controlled by the Gamemaster. (Though they are a participant in the game, a Gamemaster is usually not referred to by the technical term "player.")
Just like the previous game, survivors and zombies are NPCs.
Survivors and zombies, are, again, NPCs. Most survivor NPCs can be interacted with, to a degree. They grant the player certain quests or supply them with information (or simply babble about their experiences). Two NPCs — Hank and Kelly — are able to become survivor companions.