The document describes the key components of a hydraulic circuit: 1) a hydraulic pump that pumps oil from the reservoir and has a fixed or variable displacement, 2) a filter that cleans the oil, 3) a pressure relief valve that controls pressure, 4) a check valve that allows one-way flow, 5) a hydraulic reservoir that stores fluid, 6) a directional control valve that controls fluid flow, and 7) a hydraulic cylinder that converts hydraulic power into mechanical force. It also briefly mentions types of hydraulic circuits.