TIG stands for Tungsten Inert Gas. Tungsten refers to the non consumable electrode within the torch. Inert gas just means a gas that doesn't react with anything, in this case we use Argon. It is used to shield the weld from any bits of dirt or contamination that could weaken the weld.
Look at the photo above - it's David welding a bolt onto the end of a weathervane pole. The electrode within the torch produces a high temperature arc that creates a small pool of molten metal on the piece, into which filler wire is manually introduced. It involves having a very steady hand, keeping the torch at the correct angle and distance from the work piece, while getting into a steady rhythm of adding the filler rod with the other hand. So, unlike MIG welding, it cannot be taught quickly - it's very highly skilled but also way more accurate. There is something incredibly satisfying about producing a perfectly neat little TIG weld and it sometimes seems a shame to electroplate and powdercoat over it!
TIG welding is also a slower process and therefore not commonly used in mass production, but we honestly just love doing it, we find it a relaxing and enjoyable way to work, which is a trade off I'm happy to make for our work making us slightly less money because its slower. I do love it when we occasionally have a customer who has a good knowledge of welding & appreciates the level of skill involved!
The advantages of TIG welding are that you have significantly more control over the heat you are putting in, and therefore the metal less likely to warp. It produces virtually no spatter so the weathervane or swinging sign is much cleaner, and it produces small neat welds that are barely noticeable but incredibly precise and strong. In short, the end product is of a higher quality.
The only disadvantages really are that it takes years to learn TIG welding to a professional standard, and is a slower process, so if making a quick profit is your aim, it's probably not the type of welding for you!
We do own a MIG welder, but we rarely use it now that we're so used to TIG. I imagine the next time we get it out will be when we come to teach the twins (currently 4) to weld, which depends on their heads being big enough to fit a welding mask!