T-joint a right-angled joint, esp one in wood, making the shape of the letter T
Lap joint a joint made with two pieces of metal, timber, etc., by halving the thickness of each member at the joint and fitting them together.
Butt joint a joint formed by two surfaces abutting at right angles.
welded together using oxygen acetylene a mixture of oxygen and acetylene, used in a blowtorch for cutting steel plates or the like.
Arc welding a technique in which metals are welded using heat generated by an electric arc.
Mig. metal inert gas welding: a method of welding in which the filler metal wire supplies the electric current to maintain the arc, which is shielded from the access of air by an inert gas, usually argon.
Conclusion Oxygen and acetylene is used for thinner metals. Arc welding is for welding the thickest kinds of metal. Mig welding is used for anything in between.