MIG welding wire is a continuous, consumable solid or flux-cored electrode, commonly made of steel, stainless steel, or aluminum, used in Metal Inert Gas (MIG) welding to join metal components. It feeds through a gun, melting via an electrical arc to form a weld pool, while external gas provides protection. Common types include ER70S-6 (mild steel) and ER4043/ER5356 (aluminum).