Piping is used to convey fluids (usually liquids and gases but sometimes loose solids) from one location to another. The engineering discipline of piping design is that which gets the fluid to where you need it.
Industrial process piping (and accompanying in-line components) can be manufactured from wood, glass, steel, aluminum, plastic and concrete. The in-line components typically sense and control the pressure, flow rate and temperature of the transmitted fluid, and usually are included when one discusses the concept of piping design.
Plumbing is the form of piping that most non-technical people are familiar with, as it constitutes the form of transportation that is used to provide liquids (water) and gases (natural gas) to their homes. Piping also removes waste from the household in the form of sewage.
Piping also has innumerable other industrial applications, which are crucial for moving raw and semi-processed fluids for refining into more useful products. Some of the more exotic materials of construction are titanium, chrome-moly and various other steel alloys.