Plumber goes back to Roman times in reference to the lead they worked with.
As Dan said, plumbers work almost strictly with water and waste.
Pipefitter/steamfitter works with every other liquid/gas/substance, many of these dangerous and made more so by heat and pressure.
Much like aviation, explosives, and mining, many fitter practices and regulations are written in blood.
Fascinating field as with all the other trades.
There is a lot of romance in the trades, building things, fixing things.