Carbon steel, stainless, aluminum, and some specialty alloys. Carbon steel pipe is the most common, used for structural, irrigation, ag, and light commercial work. Stainless pipe shows up in food, brewery, and medical facility work.
Aluminum pipe runs on boats, trailers, and railing. The right process depends on the material: TIG for stainless and aluminum, MIG or stick for carbon, and stick for outdoor or dirty conditions.