Tub Shower Valve
Most of all the products built today are engineered to use the cheapest materials possible. Like most things built years ago, Tub & Shower valves were made of all brass giving them a life span of multiple generations. Tub & Shower valves have three major components: a valve body that is installed inside the wall, stems, and trim.
Stems are the moving part that allows one to open and close the valves. Trim refers to all the shiny stuff (handles and escutcheons). If properly rebuilt, you can have a brand new Tub & Shower valve that will operate like the day it was purchased.