What a Certiorari Is

A certiorari is an order or writ by which a higher court is requested to review the decision of a lower court alleging that the decision has been irregular, incomplete, or if there has been an error of law. It raises a question of law.

If the application is successful, it renders the decision at issue of no force or effect and null and void.

The special civil action for a certiorari and appeal are two different remedies that are mutually exclusive; they are not alternative or successive. Where an appeal is available, the certiorari will not prosper, even if the ground therefor is grave abuse of discretion. 

The basic rule: A certiorari is not a substitute for the lapsed remedy of an appeal.

References:
http://www.chanrobles.com
duhaime.org
https://bataspinoy.wordpress.com