In MIG/MAG welding, an arc is created with the power supply through the welding gun between the welding wire being fed and the work piece. The arc fuses the material being welded and the welding wire, thus creating the weld. The wire feeder continuously feeds welding wire through the welding gun throughout the welding process. The welding gun also provides shielding gas to the weld.

The MIG and MAG welding methods differ from each other in that MIG (metal inert gas) welding uses an inert shielding gas, which does not participate in the welding process, while MAG (metal active gas) welding employs an active shielding gas that participates in the welding process.

Usually the shielding gas contains active carbon dioxide or oxygen, and therefore MAG welding is by far more common than MIG welding. In fact, the term MIG welding is often accidentally used in connection with MAG welding.