After your first experience with threaded pipe, you'll appreciate why this material is all but extinct in new installations. Cutting, threading, and assembling steel pipe requires muscle. Sometimes when you're trying to take old pipe apart, you'll swear it is welded together.
If your home was built before World War II, its supply pipes are likely to be threaded steel. This doesn't mean you have to use the same pipe for improvements or repairs. Special fittings let you break into a line and add copper or plastic.
Black threaded pipe, which lacks the shiny gray color of galvanized, is meant for gas only, and is still commonly used. Do not use black pipe for water lines.