The diode is to electrons exactly as a check valve or one-way valve is to water. Electrons can easily pass in one direction but are prevented from traveling through the diode in the reverse direction. This is great for changing Alternating Current (AC) where electrons go back and forth inside the wire, to direct current (DC) where the electrons always go the same direction. The diode merely refuses to let the electrons go backward when the voltage reverses. There are literally thousands of other uses for the common diode (sometimes called rectifiers) other than that though.

The zener diode is a very different device. Where the common diode is designed to never let current flow in the reverse direction, the zener is designed to break down and let current flow backward at some specific voltage (electronic term for pressure) They conduct like a normal diode in the forward direction but in the reverse direction will not allow the voltage across the diode to exceed the rated zener voltage. Consequently, they make great voltage regulators or references. The current must be externally limited because when one of these turn on--they turn on hard and will be damaged if the current isn't held to a safe value. This is usually accomplished with a simple resistor.

Light Emitting Diodes (LED's) behave similarly to zener diodes in a way, except they start conducting current in the forward direction at a given voltage and emit light. With reverse voltage, they block current just like conventional diodes. Consequently, LED's are available that have two diodes connected in parallel but opposite polarity, that change color when the voltage is reversed. Most LED's are the kind you can see, but some emit invisible infrared light. Just like the zener, the current through the diode must be limited externally. This requires a little explaining. You see, a 3 volt incandescent flashlight bulb is truly a 3 volt device. Its own internal resistance will limit the current to a safe level when operated anywhere near its rated voltage. If the voltage is increased slightly, the lamp simply burns a little brighter. But a 2 volt LED is not truly a 2 volt device. It has very little current limiting ability. An increase of voltage to only 2.1 volts could probably destroy it. That explains why so many of you have lost LED's by just trying to see if they really worked. They require at least 50 ohms of series resistance for every volt at which they're operated over 2 volts to ensure they won't be damaged.

I hope this sheds some light (no pun) on this subject.