The difference between pipes and tubes is a matter of sizing. For intance, PVC pipe for plumbing applications and galvanized steel pipe are measured in iron pipe size (IPS). Copper tube, CPVC, PeX and other tubing is measured nominally, basically an average diameter.These sizing schemes allow for universal adaptation of transitional fittings.
For instance, 1/2" PeX tubing is the same size as 1/2" copper tubing.1/2" PVC on the other hand is not the same size as 1/2" tubing, and therefore requires either a threaded male or female adapter to connect them. When used in agricultural irrigation, the singular form "pipe" is often used as a plural.