In a MIG welder, the tank that connects to the welder contains a gas mixture called Metal Inert Gas, and this is where it derives its name. This creates something of a protective area around the arc when you are welding, and it can lead to much cleaner welds overall. The MIG welder is a wire-feed welder, which means that the metal it uses to crate the weld is on the inside of the welder. It feeds out through a spool. The exact type of metal on the spool can differ based on the job you are doing.