The main difference between TIG and MIG welding is that one uses a consumable wire electrode (TIG) and the other uses a non-consumable tungsten electrode (MIG). The MIG welding process produces an electric arc between consumable wire electrodes and the workpiece metals.
MIG can be difficult to describe. In general, MIG welding uses a semi-automatic or automatic arc to weld. It also uses a continuous wire electrode and shielding gas, which are fed through a lead to a welding gun (sometimes called a torch).
In TIG welding, an arc is also used to weld. The main difference between MIG and TIG welding is that MIG uses a non-consumable electrode and separate consumable filler material. As the non-consumable tungsten electrode produces an arc, you must manually feed a "rod" of filler material into the weld puddle with your second hand.