Definition of Pipe fitting:

Pipe fitting is the occupation of installing or repairing piping or tubing systems that convey liquid, gas, and occasionally solid materials. This work involves selecting and preparing pipe or tubing, joining it together by various means, and the location and repair of leaks.

Pipe fitting work is done in many different settings: HVAC, manufacturing, hydraulics, refineries, nuclear-powered supercarriers and fast-attack submarines, computer chip fabrication plants, power plant construction, and other steam systems. Pipe fitters (sometimes called simply "fitters") are represented in the USA and Canada by the United Association of Journeymen and Apprentices of the Plumbing and Pipe Fitting Industry of the United States and Canada.

Pipe fitting is not plumbing; the two are related but separate trades. 

Materials, techniques, and usages vary from country to country as different nations have different standards to install pipe.