What is earth hour?

Yesterday when I am chatting with a few of my friends, they told me that earth hour is coming soon. At this point of time, I am very confused. What is earth hour? This is the first time in my life I have ever heard of this word.

When I ask them, they do not give me a reply. When I ask one of my friend, he replied, "It means the earth's hour, you stupid!". I guess this friend is the much stupidier ones giving me such a stupid answer.

My australian friends and my malaysian friends seems to be all talking about this earth hour thing, so I guess it is something international. Since it is international, why I did not heard of it at all in my own country?

At first I thought it is omething terrible, like maybe the whole world's electricity have to be cut off. Or maybe it is some phenomenon, like for example the eclipse moon or the leap years. Or maybe it is something which is going to do with saving the world, saving the earth.

What exactly is earth hour? Can anyone tell me?