Pressure control valves are used to control and regulate pressure in hydraulic systems. The main types are pressure relief valves and pressure regulators. Pressure relief valves are connected to high and low pressure lines and are used to limit the maximum operating pressure. They contain a spring-loaded poppet that opens to allow fluid to return to the tank when pressure reaches a certain threshold determined by a formula. Directional control valves and flow control valves are also used to direct and regulate fluid flow respectively. Check valves only allow fluid flow in one direction to prevent backflow.