Welding is the process of joining to materials together. The most commonly welded material is metal which is welded using MIG, TIG or ARC welding. This welding melts a metal welding material to join the two different pieces together.
A MIG welder feeds a wire out as you weld while blowing a Metal Inert Gas at the same time, thus where the MIG name comes from. When the metal wire contacts the surface to be welded the current flows and fuses the two materials together.