NPCs (non-player characters) are neutral characters that can give you information by talking to them, and may also offer some services for money. NPCs can be recognized by their yellow foot circles.
Types of NPCs you will encounter are commoners, which offer no services but information, traders, which you can trade items with, healers, where you can buy healing and donate money to a deity, sages, which can identify and uncurse items, and trainers, which will help you advance in your chosen profession.
NPCs are not outhangers that stay at the same spot all game long. They will run around and attack monsters if they get in their way. Thus, you might want to protect NPCs whose services are valuable to you.