House

A house is a safe place where player or NPCs can spend there time without worry of attacking enemies. By definition, a house in terraria is:

Has walls (The background blocks)

Has a table and chair

A door

A light source

Once you have one set up, the guide will move into it and you have to make another house. Once this is set up, a Dryad (Only if you defeat a "Powerful monster"), Demolitionist (Only once you use dynomite), Arms Dealer (Only when you find a gun), Merchant (Once you have 50 silver) or Nurse (When you upgrade your maximum life) can move in.