Indoor plumbing is what makes civilization great. Plumbing refers to the pipes that get the water in and the sewage out. If a snake shows up in the toilet, it probably came in through the plumbing. Yikes!
You can use the noun plumbing whether you're talking about the bathroom sink, the pipes connected to your dishwasher, or the bathtub drain. A home's heating system often consists of plumbing too. The work of fixing or installing this type of system is also plumbing. The word's earliest meaning was "the weighting of a fishing line," from plumb, originally "to sink like lead," which has the Latin root plumba, or "lead."