A zener diode is designed to operate in the reverse breakdown, or zener region, beyond the peak inverse voltage rating of normal diodes. 

In the forward region, it starts conducting around 0.7 V, like an ordinary silicon diode. In the leakage region, between zero and breakdown, it has only a small reverse current. The breakdown has a sharp knee, followed by an almost vertical increase in current.

Zener diodes are used primarily for voltage regulation because they maintain constant output voltage despite changes in current.