What is zener breakdown?

Distinguish between avalanche and zener breakdown.

Why zener breakdown voltage decreases with increase in temperature?

A general purpose diode is more likely to suffer avalanche breakdown rather than zener breakdown because

Two Zener diodes having specification 12V, 1/4W are connected in series. If breakdown voltage of each diode is 5V, then what is the breakdown voltage in the series combination of the diodes?