The 4N35 is an optocoupler (or opto-isolator) that contains an infrared LED and a phototransistor. It provides electrical isolation between its input and output, protecting sensitive circuits from high voltage or noise. The 4N35 is commonly used for signal isolation, switching applications, and interfacing between different voltage levels in electronic systems. It typically features an isolation voltage of 5 kV and can handle a range of input currents, offering reliable performance in various applications requiring electrical separation and signal transfer.