Via Commerce Metals:
Tubing is generally used for structural purposes and the OD [outer diameter] is an important and exact number. Tubing size is specified by OD and the wall thickness (WT).
Pipes are categorized as tubular vessels used in pipeline and piping systems, and commonly transport gases or fluids. They are specified by "Nominal Pipe Size" (NPS) and Schedule (wall thickness).
Obligatory: This is not a pipe: