MIG welding (formally Gas Metal Arc Welding, GMAW) feeds a continuous wire electrode through a hand-held gun. An electric arc between the wire and the parent metal melts both, and a shield of inert gas (CO2, argon, or a mix) keeps oxygen away from the weld pool.
MIG is the workhorse of Australian fabrication. It's fast (you can lay a metre of fillet in under 60 seconds), forgiving (the wire feeds itself, so the operator only needs to control travel speed and angle), and clean (no slag to chip off afterwards). Most gate-fab welders run a 0.9 mm ER70S-6 wire on Argoshield 51 (Ar/CO2 mix) for general steelwork.
Limitations: outdoor or windy conditions blow the gas shield away and contaminate the weld. Reach for stick or flux core in those cases.