For //welding//, 100% Argon, Argon+1-2% hydrogen, or 60%argon 40% helium, to protect the weld puddle from oxidation.
For cutting, a number gasses are used.
Most common is compressed air. This works very well for carbon steels. For cutting at high speeds on carbon steel of modest thickness <3/4", pure oxygen may be used. Pure O2 tends to significantly lower the life of consumables and electrodes, but the cost of consumables is fairly small compared to cost savings created by being able to increase cutting speeds.
For stainless steels, and for high-current cutting of thicker steels in general, it's common to use pure nitrogen. This results in a much cleaner cut with very little oxidation which is very desirable for stainless steels. However the cutting speed is typically lowered using 100% N, compared to compressed air.
For applications were very clean cuts with zero oxidation is necessary, and on very thick stainless or non-ferrous alloys, dry hydrogen or argon+hydrogen is used. These are more expensive than the other gases mentioned.