TIG and MIG welding are similar. In TIG welding a filler rod is held in one hand and the torch in the other. It uses a non-consumable tunsten electrode to strike the welding arc.
In MIG welding, the electrode is metal wire fed off of a spool automatically through the torch. The wire is the filler material. It only requires one hand.
With TIG, if the sheet metal joint is really tight (very difficult to achieve with replacement panels), filler rod may not be required since the two panels can be fused together and any depression remaining can be filled with lead or bondo prior to painting.
Most people use MIG to weld in replacement auto panels since it is generally considered easier and faster. I prefer TIG since I can use the filler rod to build up the seam without burning through the thin sheet metal panel. The weld can then be ground down using a small grinder/sander flush with the adjoining panels.
I learned how to TIG weld at CCRI (Community College of Rhode Island).