Hydraulic valves and pneumatic valves both control the flow of fluids or gases in machinery, but they operate in different systems. Hydraulic valves regulate the flow of hydraulic fluid, which is a type of oil, to control the movement of heavy equipment like bulldozers or cranes. On the other hand, pneumatic valves manage the flow of compressed air to control the operation of lighter machinery, such as pneumatic drills or air compressors. The main difference between the two lies in the type of fluid they control—hydraulic valves handle oil, while pneumatic valves handle air.