A diode is a semiconductor that only allows current to flow in one direction, it is a one way valve for electricity.
Diodes come in all shapes and sizes, the most important difference in them is the current rating, i.e. how much you can push through without overheating and burning the diode up. Here I demonstrate different diodes.
Here I show how to test a diode. Testing is done with a multimeter, an analog multimeter is preffered.
Here is some information on the numbers you may find on a diode.
Specifies semiconductor material
Specifies type of device
Diode – low power or signal
Diode – variable capacitance
Transistor – audio frequency, low power
Transistor – audio frequency, power
Transistor – high frequency, low power
Diode – sensitive to magnetism
Transistor – high frequency, power
Switching device, low power, e.g. thyristor, diac, unijunction
Transistor – switching low power
Switching device, low power, e.g. thyristor, triac
Transistor – switching, power
Surface acoustic wave device
Diode – voltage reference
The characters following the first two letters form the serial number of the device. Those intended for domestic use have three numbers, but those intended for commercial or industrial use have letter followed by two numbers, i.e. A10 – Z99.