Tailed Beast

The are nine giant demons that differ by the number of tails they have, with each possessing an amount from one to nine. They are large, living forms of chakra, sometimes referred to as, giving them power that far outmatches most shinobi. However, this immense strength is tempered due to their bestial nature, which restricts them from being intelligent enough to use it effectively. Tailed beasts sealed within humans are able to benefit from the knowledge of their hosts, giving them the ability to speak and better utilize their powers. These hosts are known as jinchūriki. Akatsuki has been capturing the tailed beasts and their hosts, and currently possess seven of the nine.

History
The tailed beasts have been around since long before the founding of the ninja villages. After the First Great Shinobi World War, Hashirama Senju, the First Hokage, managed to capture and control some of them with his Wood Release techniques, and used them as peace treaties to the other great shinobi nations in order to stabilize the balance of power between them. After his death, however, the shinobi nations couldn't control these giant demons themselves and they began to run wild, so the nations instead moved to seal the tailed beasts within humans, which they were called jinchūriki.

One-Tailed Shukaku
It has been captured by Deidara then sealed by Akatsuki.

Two-Tailed Monster Cat
It has been captured by Hidan and Kakuzu then sealed by Akatsuki.

Four-Tailed Beast
It has been captured by Kisame Hoshigaki then sealed by Akatsuki.

Five-Tailed Beast
The is a demon sealed within an unnamed shinobi from Iwagakure. It has been captured and sealed by Akatsuki.

Six-Tailed Beast
The is a demon sealed within an unnamed young man of unknown origins. It has been captured and sealed by Akatsuki.

Trivia

 * On the third of July 2009, the second Naruto artbook will be published. According to advertisement, it will "finally reveal the mystery of the tailed beasts."
 * Out of the known tailed beasts, the Seven-Tails and the Eight-Tails do not actually have tails. The Eight-Tails has eight cephalopod arms for tails while the Seven-Tails has seven insect wings connected to the tip of its abdomen.
 * According to some fans, the Legend of the Tailed Beasts is a story from Japanese folklore which Naruto author Masashi Kishimoto draws from to create the tailed beasts in his work. However, it is an example of internet-based fakelore which originated from this Japanese site. The veracity of the legend breaks when it was cross-referenced with non-fandom sources on Japanese folklore. For example, the isonade, which was thought to be the Three-Tails, is in fact equipped with only one large, hook-covered tail; and the hōkō, which was thought to be the Five-Tails, actually has no tail at all. Additionally, the author of the story admitted it was not a true legend, unfortunately it has gone largely unnoticed by most fans of the story.