What is the job of a diode?

Answer 1

A diode is used primarily as a Rectifier

Answer 2

Rectification of an AC sinewave, or in the case of a zener diode, voltage regulation.

A diode acts as a one way valve. Allows current to flow one way and block the other way.

DC current will pass one way. AC current will pass during one half of the wave form and nothing during the other half, giving pulses of half wave recification.

While the diode is blocking (reverse bias), if you increase the voltage it will reach a point where it suddenly collapses and passes all the current.

This is normally fatal to an ordinary signal signal diode.

Zener diodes are made, to recover. The voltage at which they collapse and pass current, can be precisely manufactured. Zeners can now be used for voltage reference and regulation.

Answer 3

They only allow current to flow in one direction.