Many different kinds of diodes are available. Here are some of the major types:
Small signal diodes are used to transform low current AC to DC, detect (demodulate) radio signals, multiply voltage, perform logic, absorb voltage spike, etc.
Functionally identical to small signal diodes, power rectifiers can handle much more current. They are installed in large metal package that soak up excess heat and transfer it to a metal heat sink. Use mainly in power supply.
The Zener diode is designed to have a specific reverse breakdown (conduction) voltage. This means Zener diode can function like a voltage sensitive switch. Zener diodes having breakdown voltages (Vz) of form about 2-volts to 200-volts are available.
All diodes emit some electromagnetic radiation when forward biased. Diodes made from certain semiconductors (like Gallium Arsenide Phosphide) emit considerably more radiation than silicon diodes. They're called light-emitting diodes (LED's).