All about Diodes

A diode is a very early type of semiconductor. It allows electricity to flow in one direction, but blocks it in the opposite direction. (A light-emitting diode is a much more recent invention.) Like an LED, a diode can be damaged by revers- ing the voltage and applying excessive power, but most diodes generally have a much greater tolerance for this than LEDs. The end of the diode that blocks positive voltage is always marked, usually with a circular band, while the other end remains unmarked. Diodes are especially useful in logic circuits, and can also convert alternating current (AC) into direct current (DC).

A Zener diode is a special type that  blocks current completely in one direction, and also blocks it in the other direction until a threshold voltage is reached.

Signal diodes are available for various different voltages and wattages. Like any semiconductor, they can overheat and burn out if they are subjected to mistreatment. The schematic symbol for a diode has only one significant variant: sometimes the triangle is outlined instead of filled solid black (see in Figure below).