The diode is an non-linear (current vs. voltage not proportional) electronic component of the semiconductor class. It is a two leaded device that is primarily used to block the flow of current in one direction. The terminals of the diode are called the anode (positive) and the cathode (negative).
In the reverse mode, the diode will block current until the breakdown voltage is reached, then the device is destroyed. In the forward mode (conducting mode), the diode let the current flow and will drop some volts, this is called the forward voltage drop. In datasheets, this value is abbreviated Vf.
There are multiple diodes types;
This diode is the simplest of all. Some of them can carry only small current values (usually signal diodes). Some others like the rectifier diodes can carry larger amounts of current. Some of the most popular diodes are part of the 1N4000 series (1N4148; 1N4007; etc...). For this type of diode, you often see forward drop of 0.7 Volts.
Schottky and Zener
The Schottky diode is usually used where you need a small power dissipation. This is due to their low forward drop (often 0.2 Volts). The Zener is a special type, where the reverse mode will conduct at low voltage values : 24V, 12V, 5V, 3.3V etc... This is useful to create voltage references and small current capacity regulators.
The ubiquitous LEDI think everyone is familiar with the LED (Light Emitting Diode). This type of diode is available in a lot of different colours, shape and brightness. The most common is the discrete 5mm red LED. Here is what it looks like :
On this image, you can also see the schematic symbol of the LED and how to identify the terminals. For through hole parts, the longer lead is the anode. The forward voltage drop of the red LED is about 2 Volts. Other colours have different forward drop. To light the LED, current has to flow from it's anode to cathode. Here is the basic connection schematic :
I hope that diodes have no longer secrets for you!