ok, you asked , so here it is.
mig welding, uses gas and wire, to weld two pieces together.
the two pieces are held together, only by the weld.
mig on the other hand, is like "brazing". it litterally melts the two pieces together.by creating a puddle, and adding only enough "stick" to keep the puddle "alive", you infact, make one piece out of two.
mig is slower, by far, than tig, but much more durable, and generally leaves a better looking "meld"!
just jt