The nine-tailed fox had nearly destroyed Hidden Leaf 12 years previously. It is one of the "bijuu", a tailed beast of the ancient world. It took the sacrifice of the Fourth Hokage to imprison the demon and place it into the body of the young baby Uzumaki Naruto. It was the Fourth's hope that the Leaf villagers would look at Naruto as a hero for serving as the vessel to contain the demon. However, the people of leaf village see Naruto as the Kyuubi itself and shun him.
The power of the Kyuubi has affected Naruto in several ways physically. When Naruto's body becomes injured, the power of the Kyuubi will leak out and heal Naruto's body at a very quick pace. Also, when Naruto experiences emotional experiences the chakra and power of the Kyuubi begins to leak out and turn Naruto into a more feral individual. At first he had little control over the ability to draw on the Kyuubi's chakra, but the Sannin Jiraiya taught Naruto how to reach deep inside himself and convince the Kyuubi to give him extra chakra when needed. This large amount of additional chakra allows Naruto to perform Kage Bunshin on a very large scale and also summon the Toad Boss Gamabunta.
Now that Naruto is dead though, no one knows who has possetion of this demon now.
