Kyuubi is a demon fox. Where he came from no one knows. He came to the Leaf Village 12 years ago and destroyed most of it. His tail could cause earthquakes and tsunami's. His chakara ("chakra" in the English version) is everlasting and very powerful. It was when the 4th Hokage sealed the Demon Fox inside a young boy as a vessel and thus died. This boy is Naruto Uzumaki who contains the demon fox, though he doesn't realize it