A plumber is someone whose job includes fixing pipes and installing water and sewage systems. If your kitchen sink is clogged, a plumber is the person to call.
Plumbers's work ranges from relatively simple tasks like repairing toilets and removing clogs from drains, to much more complicated jobs including replacing all the old corroded pipes in a bathroom floor with new ones. The word plumber has been around since ancient Rome, when a plumber was "anyone who works with lead," and lead was a common material for pipes, baths, drains, and conduits. Fixing anything made of lead (which included roofs) made someone a plumber back then.