So, what does MIG mean anyway? Before we get there, let's talk about arc welders. Arc welders use high voltage electricity to generate enough heat to make a weld. There are different types of arc welders -- stick, TIG, MIG -- but the difference between them is not in the electricity they use or how they use it, but in the other element common to arc welders, a gas shield. The gas shield can be created by a flux that releases gas due to a chemical reaction, or by a cloud of gas released from a tank connected to the welder. In the case of a MIG welder, the tank is filled with a mixture named Metal Inert Gas by the industry. The gas recipe varies, but the name indicates that none of them will react with metal and add any contaminants to your weld. This gas is pumped through your welding cable from that metal tank you had to lease or buy. It comes out of the same nozzle your welding wire is fed through so it literally creates a protective cloud around the arc as you're welding.
A MIG welder is also a wire-feed type welder. The metal that it uses to create the weld material is held on a spool inside the welder. The type of material it uses depends on what type of metals you're joining, but it's always a metallic wire. For beginners, or for welders who need ultimate portability, some welding wire contains flux inside it, eliminating the need for a separate tank of welding gas. This works, but is inferior to a proper gas setup. The wire is fed through the nozzle coming out as you pull the trigger. The welding wire itself completes the arc that was started when you clamped the other electrode to your welding project.
A MIG welder has a number of different heat settings which allow you to set the machine to just the right power to get a deep weld with good penetration, but not so much power that you burn a whole in your project. Don't worry if you do this a few times before you get things right. Even seasoned welders are suprised from time to time and end up having to make last minute adjustments to their heat settings. There is also an adjustment to the feed rate of your wire. This will vary by project and equipment, but as you get to know your usual jobs and your welding machine, you'll fine tune your feed rate. It's always a good idea to do a test bead on some scrap metal before you start working on your valuable project. A properly set up machine that is welding clean metal will sound like bacon sizzling in a pan. Getting the heat and feed settings right before the real job is in front of you can save lots of time and money.