Gas Metal Arc Welding (GMAW), often times referred to as Metal Inert Gas (MIG), is a semi-automatic or automatic welding process. In this process you use a consumable wire electrode that is continuously feed through a wire gun along with a shielding gas to protect the weld. With the GMAW process you use a constant voltage, DC power source most of the time. Constant Current Systems, as well as AC can be used for some metals.
The equipment involve in GMAW is a welding gun, a wire feed unit, welding power supply, an electrode wire and shielding gas supply.
The wire electrode is feed from the wire feed unit into the liner of the welding gun. The liner keeps the wire from binding up inside the gun while being feed to the work piece. As the wire passes through the contact tip it becomes electrically charged. Once the wire touches the grounded work piece it then makes the arc and you can begin welding. As you are welding, the gas nozzle is used to evenly distribute the shielding gas over the weld zone. The larger the nozzle the more shielding gas will be distributed over the weld area. So, if you are going to be using wider weld beads then a larger nozzle would be needed.
GMAW torch nozzle cutaway image. (1) Torch handle, (2) Molded phenolic dielectric (shown in white) and threaded metal nut insert (yellow), (3) Shielding gas diffuser, (4) Contact tip, (5) Nozzle output face
The major advantage that GMAW has over other welding processes is that it is fast and pretty clean. You rarely have to stop with the GMAW process too. Where as in GTAW or SMAW you have to stop when you run out of filler rod.
On the flip side, it is difficult to use in hard to reach places because of the size of the nozzle.