A pipe is usually defined by its internal diameter unlike a tube that is defined by its external diameter. Both the pipe and tube are available in different wall thicknesses. A pipe however is most commonly available in only two thicknesses defined by schedule 40 and schedule 80. A pipe's primary purpose is to facilitate fluid flow. The pipe diameter and thickness is selected considering the following factors. The amount of fluid flow it needs to allow, the available pressure capacity of the fluid pump and the required pressure differential to get the fluid from point A to point B over some distance and altitude. A schedule 80 pipe is thicker and is used when the fluid pressure is higher. A larger diameter is used when the flow is higher and the available source pressure is limited. Sometimes, a schedule 80 pipe may be unable to withstand the higher pressure, so a larger diameter pipe may be chosen to reduce the pressure differential. Conversely, a relatively lower pressure differential would mean that a schedule 40 pipe is used. Pipes are available in many materials such as PVC, Polypropelene, Steel (usually galvanized for corrosion resistance) and aluminum (usually annodized for corrosion resistance).

A tube is traditonally used for non-fluid flow applications usually in some type of structural or non-structural fabrication. Although, a round tube is sometimes used to facilitate flow. A tube is usually made out of steel and aliminum. It is also available in other shapes such as a square or a rectangular tube.

http://www.wermac.org/pipes/pipe_vs_tube.html

A pipe is measured in terms of its internal diameter while a pipe is measured in terms of its external diameter