Machine mart and salesmen not knowing what they talk about, hmm surley not
I have just about every welding machine going in my workshop and i have welded allsorts of material and of various thicknesses.
I have only glanced through this thread quickly and can see that most of the questions have been answered already.
Brazing, similar to solder, whereas the material thats being joined does not get melted, result a fairly strong joint.
Arc, is a welding rod that has a flux coating around it, as soon as the metal core touches the material to be joined, it starts to melt both material to be joined and the rod, the flux also melts creating a protective coating over the weld whilst it cools and solidifies.
Arc is used mainly for thick plate type steel and cast steels.
Mig, (metal inert gas) is a welding wire which is fed off of a reel and through a gun, its is accompanied by a shrouding gas to keep the weld pure from the atmosphere. The gas is non flamable and comes in a variety of different mixes for different applications. Mig can be used for stainless steel and also for alluminium alloys.
Tig, (tungsten inert gas) is an electric flame produced by a tungsten rod which is held in a gun, shrouded by pure argon gas, this type of welding is the purest weld that you can have. Tig welding can weld all types of metal, from mild steel, stainless steel, copper, alluminium alloys and even titanium.
It is also my favourite type of welding.
Gas welding, (oxygen/acetelyne) is a gas flame from a nozzle by mixing the two gasses together, which burns hot enough to melt the material being welded. A filler rod is then melted along side the material to fill the welded joint.
All welded joints are far stronger than any form of solder or brazed joints, simply becuase the material is melted together.
Hopes this helps a few understand the various types of welding techniques.