Diodes easily conduct current in one direction (forward biased) but not the other (reverse biased). The exception being zener (sometimes called avalanche) diodes, which still passes current while forward biased, but can also safely pass some current while reverse biased, once their zener voltage is exceeded.
Rectifier diodes are made especially to rectify (only allow current in one direction). They still have limitations though of course. They usually still block about 0.7V while forward biased, and have a limit to how much voltage they can block while reverse biased. Make sure to check the datasheet for the part number you are using, to verify that it can block the voltage you will be using, and can handle the current you will demand of it. Covered in more detail at – Rectifier diode
LED stands for Light Emitting Diode. LEDs are relatively poor diodes. They are really only intended to produce light wile forward biased. They block more voltage while forward biased than other diodes, typically about 1.5V to 3.2V.
LEDs should be limited to 20mA or less unless otherwise indicated. Covered in more detail at – Light Emitting Diodes – Indicator LEDs
Zener diodes are used while reverse biased (RB). They block a certain voltage which depends on their rated zener voltage. Much like resistors come in a wide range of values, zener diodes come in a wide range of values.
Current starts flowing through the RB zener diode once it’s zener voltage is reached. This current is usually sent directly to the negative supply (ground). The zener diode can be thought of as a voltage release valve because it prevents a voltage from exceeding the zener voltage.
Schottky diodes are used just like rectifier diodes, but with the benefit of blocking less voltage while forward biased. Typically about 0.3V instead of approx. 0.7V. This results in less power loss, but schottky diodes don’t block reverse voltage quite as well as rectifiers.
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