What is a Nuclear Reactor Meltdown?

There is serious concern over a nuclear reactor meltdown in Japan. Please could you tell me what it is?
I'm scared because I have no idea what a nuclear reactor can cause to the planet, I've been told it may affect the whole world.
I live in California, and I fear that some radioactive materials may get here and make us sick.

Explain to me what is a nuclear reactor meltdown and if it happens, how many fatalities are expected?

A meltdown is severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping.
In nuclear power plants, coolant is normally used to keep the reactors at a low enough temperature so that a nuclear reactor meltdown does not happen.

If there was very bad nuclear meltdown, some radioactivity could possibly drift through the air to America.
If it happens, people should stay inside, close windows and air-condition and if you go out wash your clothes and shower very well.