A diode acts as a rectifier, permitting current to flow only in one direction. A diode has two electrodes, one marked as positive and the other negative. The diode allows current to flow from positive to negative terminals, and blocks any curent in the reverse condition. A diode with a higher voltage on the negative terminal is known as a reverse biased diode, and no current flows until the voltage is so large that the diode breaks down (typically, a non destructive repeatable condition).
There is a minimum voltage necessary for the diode to allow current to flow, known as a threshold voltage, and a small voltage drop across a diode when current is flowing. For silicon semiconductor diodes, this voltage is near 0.7 V.
There are a large variety of diode types. Common types are schotckey diodes, Zener diodes, and PIN diodes. Diodes can be made to be radiation sensitive (photodiodes) or light emitting. LEDs are light emitting diodes, and there are also laser diodes.
Some applications of diodes are AM radio demodulation, power conversion of AC to DC current, over-voltage protetion, logic gates, radiation detection, or as a temperature gauge.