The Craftsman is the NPC which will sell you all of you different colors of glass and stained clay. These are good for a nice new look inside of your cell.
The Blacksmith is the NPC where you get your basic pickaxes, swords, and armor. These will contain up to gold tools and armor, stone tools and leather and wood tools.
The carpenter has all of your wood block needs. He also has your lighting for your cell and your doors. This NPC is a major NPC for designing your cell as he contains your Guards too.
The Ore Generators NPC
The ore generators are kept inside of this guy and have all of your crude ore needs as well. However, this NPC has a little deal with Slayer so you can run the command /gens to get his shop up anywhere you want.