Basics

Most modern forms of welding use some form of Shield Gas to protect the Weld Pool from the atmosphere
In the simplest form this is a pure Inert Gas
In some instances, various aspects of the weldment can be modified via using a gas mixture, often with a Reactive Gas
- In the older terminology (Pre-GMAW ), there was Metal Inert Gas (MIG) and Metal Active Gas (MAG)

How They are Utilized

Shield Gas on the Weld Torch / Welding Gun
Purge Gas (Welding) ( ~pg 628 of Modern Welding )and Secondary Shielding (Welding) and Backing Shielding / Backing Gas ( ~pg 595 of Modern Welding ) (only really needed for Titanium Welding and other odd metals/alloys
Can also be used in essentially a Glove Box but for welding ~pg 282 and 596 in Modern Welding (Called it a Controlled Atmosphere Chamber ?)
This page will not cover gasses used for Weld Heat Treatment which are typically just the same as one would use for an Oxy-Fuel Torch / Air-Fuel Torch