The difference between pipes and tubes is simply in the way it is sized. PVC pipe for plumbing applications and galvanized steel pipe for instance, are measured in IPS (iron pipe size). Copper tube, CPVC, PeX and other tubing is measured nominally, which is 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.
Most plumbers, pipefitters, and steamfitters learn their trade through a 4- or 5-year apprenticeship. Apprentices typically receive 2,000 hours of paid on-the-job training, as well as some classroom instruction, each year. In the classroom, apprentices learn safety, local plumbing codes and regulations, and blueprint reading. They also study mathematics, applied physics, and chemistry. Apprenticeship programs are offered by unions and businesses. Although most workers enter apprenticeships directly, some start out as helpers. The Home Builders Institute offers a pre-apprenticeship training program in plumbing and other trades.