Plumbing refers to the systems of piping necessary to move potable water and natural gas to and through the home, while removing gray water and sewage waste away from your home.
Some time ago, perhaps during a terrible drought, or an ice age or something equally desperate, one of our clever ancestors came up with the idea that having a stream running through his/her cave would serve well to bring in fresh drinking water and flush out waste without all the effort of hauling buckets in and out. Especially since buckets had not been invented yet. This worked well so long as your cave was up-stream from everyone because sewage treatment plants had not yet been invented either. Life was rough in those days.
Since not every cave had a suitable stream running through it, our (even cleverer) hominid wife demanded that her mate do a bit of remodeling to install one. Thus, the art of plumbing (as well as all construction) was born and continues to evolve as we speak.
A plumbing system these days consists of a water meter or well (to replace the stream), pumps (to make sure the water flow is consistent), water, drain and gas piping (to place the water/gas where we want it), fixtures (provide a place to use the water), fittings (allow us to turn the water/gas on and off), filters (to remove unwanted material from the water), water heaters (to adjust the temperature of the water) and septic lines (so we can safely live downstream from one another). And, our clever hominid/inventor is now a plumber; trained, practiced and licensed by the State to install, service, repair and modify these systems.
Plumbing piping is mostly buried within the structure of the home and is therefore difficult to access for repairs once installed. Hence, any error can be damaging and costly, so it is critical that this work be done properly and durably from the beginning.
Medieval homeowners took quality work very seriously and failures often lead to execution of the installer. This forced the trades to improve and quality skyrocketed to high levels. Unfortunately, once civilization began to creep in and the enforcement of standards was relaxed, these quality levels were never to be seen again. Now the best skills of the trades such as soldering, pipefitting and neatness have been replaced by finger pointing.