Pipes are made to industry standards. Diameters and wall thickness has been established for many different pipe sizes.<br>
Tubing can be of any size and wall thickness. Only the need will dictate the availability of a tubing size.
Also, pipes dimensions can vary more so than tubing. Tubing tends to meet stated dimensions more accuratly and is better than pipe for fabricating components in which one tube is meant to slide within another.
Aside from the other responses, I also tend to think of "tubes" with respect to heat exchanger or process heater applications, or as parts in instrumentation pneumatic or hydraulic systems. Pipes convey fluid from one equipment item to another.
