The part of the watch that makes the watch "move" or function. The movement of a watch can be compared to a car engine; without the engine, a car will not run. There are 4 types of movements: manual, automatic, quartz, and auto-quartz. Manual and automatic movements are mechanical, which means they are both made up of only mechanical parts like gears and springs. Quartz and auto-quartz have an electrical circuit and require a battery to run, but they may also have some mechanical parts.
Mechanical watches are far more expensive than battery-powered watches because they are much more labor intensive to build. Even though battery watches are inherently more accurate, almost all collectors and connoisseurs prefer manual or automatic as these movements represent the accumulation of almost 600 years of refinement, expertise, and craftsmanship.