Fire Doors can be several different kinds of doors. They can be wood or hollow metal, and if you have really deep pockets they could even be fiberglass. Most people who refer to fire doors mean hollow metal doors, and this is the cheapest way to produce a fire door.
The thing to remember is that a fire door goes into a firewall. The rating of the door and frame must be two thirds the rating of the wall. If you have a two hour firewall you need a 90 minute fire rated door. We carry Daybar Hollow Metal Doors which are fire rated. We keep in stock 18 and 20 gauge doors. The frames are 16 gauge for durability and they come embossed with a fire certification standard.
If your need information on safety fire inspections, visit the Annual Fire Inspection Details.