What is a 'bomb cyclone'?

A powerful storm is bearing down on the West Coast and bringing with it a scary-sounding weather term - bomb cyclone.

Bomb cyclone is a term used by weather enthusiasts to describe a process that meteorologists usually call bombogenesis. It's the rapid intensification of a cyclone in a short period of time, and it can happen during powerful storms such as the one northern California and the Pacific Northwest are preparing for this week.

But what is a bomb cyclone, why does it happen and why are these big storms so feared?