What is the different between certified welder and certification welder?

A certified welder is one who has successfully passed the requirements of an industry specification for welding in a particular process (SMAW, GMAW, FCAW, GTAW), and in a specific position (1G, 2G, 3F, 6G, etc.), with a specific material (mild steel, aluminum, stainless, etc.) The certification relates only to the specification that was qualified to, in the position qualified, on the material qualified, and only with the process used.

A master welder is one who can, or has, passed the most restrictive position (Usually 6G), in all processes, and in many different material groups. There is no mention of "master welder" in welding specifications, it is usually an internal company designation. It is expected that a Master Welder can weld any material, in any position, with any process.