Argon and helium both can be used to weld a range of metals from mild steel, stainless steel, aluminum, and titanium. Argon has better arc stability then helium, in otherwords the electrons flow smoother through the argon gas creating a stable arc. Argon also is better for starting an arc. Most smaller welders use argon c02 for cost because argon is expensive. The welding shop i work in uses all argon. Helium is more expensive but forms a hotter arc and alows increased weld speeds and better penitration. pure argon welding for mig or tig is the best. Additives like co2 is just for pricing but they do conaminate the weld.

Oh as to your question how is it differnt. The pure gas is just cleaner. Means less spray and less slag all over work piece and down ur sleeves and boots. <img src="/forums/images/graemlins/kewl.gif" alt="" />

Also as for mig welding a monkey can learn how to do it the hard part is learning how to tune your machine to get the desigered bead contour. Practice practice practice thats the only way to learn. When ur striking an arc look at the outside edges of your weld puddle and walk the puddle side to side while progressing down the weld area. Get a bucket of water and just dunk ur piece in it after uv laid a bead to cool it off. dont weld on hot metal it messes with ur next bead. Cool it off and start again. Make sure u grind the weld zone down to shiny metal to get a good clean weld.

Pretty soon u can get welds like this