Updated: The New York Times has an incredibly illustrative infographic about how a meltdown occurs, and how a reactor shuts down. It's a must see.
Original post: As the situation at the Fukushima district nuclear power plants in Japan has escalated from issue to emergency, a lot of people have been asking for a basic explanation of what exactly a meltdown is. This Reuters video—specific to the Fukushima plant—was the best, most clear and simple description that I could find.
(A quick reminder that we're constantly updating our list of ways to help and support the rescue and relief efforts in Japan.)
No doubt, you have more questions about the tragic situation unfolding in Japan. I've personally found this Union of Concerned Scientists "Fact Sheet" (PDF) to be invaluable. It gives some background on nuclear accidents and crises, and breaks down the terminology used to describe them. In fact, the UCS's entire "All Things Nuclear" page is a resource worth bookmarking as you follow along and try to make sense of what's really happening in Japan.
Treehugger's Michael Graham Richard also tackles some "Frequently Asked Questions"—Can Japan's nuclear power plants explode like a nuclear bomb?; Is a repeat of what happened at Chernobyl likely? What kind of radiation exposure can be expected, and what are the potential health effects?—in his very helpful post.
Speaking more generally of earthquakes, Kiera Butler at Mother Jones talked to Morgan Moschetti, a research geophysicist at the United States Geological Survey, to get answers to a whole bunch of other basic earthquake questions that you might be afraid to ask.
Back to the nuclear emergency, to better understand how a dire situation can turn into a full-blown disaster, the obvious case study is Chernobyl. This interactive slideshow by RIA Novosti is probably the best and most comprehensive explanation I've ever come across. (Thanks, Ann!)
Image: Fukushima Dai-ichi after March 14 explosion in reactor No. 3., from Digital Globe (PDF).