In operation, an internal combustion engine, such as the one you likely have in your car, needs to take in fuel and air. After burning them, it has to eject waste gases. It must do this while maintaining a sealed cylinder which can take advantage of the high pressure generated by the combustion. The solution is provided by the valves, which open and close as needed. The valves are the gatekeepers to the kingdom of the engine, controlling what goes in and out when. The valve train is the entire apparatus of rods, cams and springs dedicated to the operation of the valves. Understanding the valve train is absolutely fundamental to knowing how an engine works. This handy video gives a great introduction to the valve train, with nice animated cutaway views.