The first three are all variations of arc welding. The basic electric arc system uses a holder with flux coated rods; the flux is vaporised by the arc heat and protects the molten metal from oxygen in the air. It's a common general purpose system for iron & steel etc. As the rods can be any thickness required, it can be used at very high currents and for very thick metal. MIG (not mgi) uses filler metal fed as wire from a spool, with an inert gas flow around it (typically argon). It's similar uses to basic arc, but as the filler wire has to be fairly thin several passes may be needed to create a large fillet. TIG uses a tungsten electrode to produce the arc, plus inert gas to protect it. Separate filler rods are used to add metal. It can be used to weld aluminium and other metals that cannot easily be welded by other means. Oxy acetylene ('Gas welding') uses a gas torch to produce an extremely hot flame. Separate filler rods are used to add metal to the joint. By adjusting the gas flame mix slightly, it can be made oxidising or carburising, affecting the final properties of the weld. The same torch can be used for brazing & silver soldering, using various brass, bronze or silver alloy rods either with a flux coating or separate fluxes. Hot air is generally only used for low melting point materials like plastics - at metal-melting temperatures it would promote oxidisation or cause some metals to ignite. Edit - a few links / images added: Arc welding. MIG - note the gas cylinder and big reel of wire on the back of the machine. TIG welding with a filler rod. Gas welding.