NPCs (PHB)

Player characters cannot fight, survive, wheel, deal, plot, or scheme without interacting with nonplayer characters (NPCs). Indeed, the very heart of the AD&D game is the relationship between player characters and nonplayer characters. How the player characters react to and treat NPCs determines the type of game the group plays. Although many choices are possible, players quickly find that consideration and good treatment of NPCs is the most frequently successful route.
An NPC is any person, creature, or monster that is controlled by the DM. Most NPCs are either people (intelligent races that live in local society) or monsters (intelligent and unintelligent creatures that aren't normally found in towns and villages). The term "monster" is only a convenient label. It doesn't mean the creature is automatically dangerous or hostile. Likewise, NPCs who are people aren't uniformly helpful and cooperative. As with all things, the range of possible reactions of NPCs to PCs covers the entire spectrum.
In the course of their adventures, player characters will be most concerned with three groups of NPCs: hirelings, followers, and henchmen. It is their aid that helps player characters vanquish deadly monsters and accomplish mighty deeds. As their names imply, these NPCs can be persuaded in various ways to join the player characters in their adventures. The most common methods of persuasion are money and loyalty.