What is a nuclear meltdown?

Japan Nuclear Reactor Meltdown fallout map!

A brief and just departure from "Celebrity World":

After the 9.1 magnitude earthquake and tsunami in Japan, the world is watching the Fukushima nuclear plants that were damaged in the devastation following the events in Japan. Tom from WLUK television takes us inside a nuclear reactor and a look at what exactly happens to cause a nuclear meltdown.