MIG welding, also called GMAW, extrudes a metal wire electrode from a gun held by the welder. Power is applied to the gun by a power supply that attempts to regulate voltage at a preset level set by the welding operator. The gun also carries a shielding gas to the nozzle of the gun. A trigger on the gun turns on the gas via a solenoid in the power supply and engages the contactor, also in the power supply. Current flows down the gun through the arc and back to the power supply via the ground clamp.
This process has become very popular in the last 40 years because of its speed and ease of use.
In this photo, I am at my welding bench, using my Mig welder to fabricate a mount for a Ham radio mobile antenna.
Mig welding is very popular, because it is easily learned and because you can do and make many things with it. People are always asking if I know where they can get a good MIG welder. A good machine costs at least $600 to $700. You can buy a cheaper machine, but you'll get what you pay for. Now the nice thing about a good, small machine is that you can plug it right into 110-V outlets in your garage. However, make sure you have the proper sized circuit-breaker installed in your breaker-panel.