The 817B is an optocoupler (also known as an opto-isolator) that includes an infrared LED and a phototransistor in a single package. It provides electrical isolation between input and output, protecting sensitive components from high voltages or transients. The 817B is commonly used in switching applications, signal isolation, and interfacing between different voltage levels in electronic systems. It operates with a wide range of input currents and provides reliable isolation with a typical isolation voltage of 5 kV.