Common iron pipe is steel. The fittings are often referred to as black iron but are usually make of malleable iron (not quite steel, but better than iron). Galvanized is also made from malleable iron but has been dipped in zinc to help prevent rust.
Avoid welding galvanized pieces as they will release poisonous fumes.
Common pipe wall thickness is referred to as schedule 40. Heavy duty, high pressure pipe is referred to as schedule 80.