A diode is a semiconductor device that allows current to flow in one direction while blocking it in the opposite direction. It is made from materials like silicon or germanium and has two terminals: anode and cathode. Diodes are commonly used in electronic circuits for tasks such as rectification, which converts alternating current (AC) to direct current (DC). Diodes come in various types, including Zener diodes and light-emitting diodes (LEDs). A Zener diode is designed to allow current to flow in the reverse direction when a specific voltage is reached, making it useful for voltage regulation. LEDs emit light when current passes through them, and they are widely used in displays and lighting applications.