Extremely powerful tailed demons. Sealed within Naruto is the nine tailed fox, the most powerful of the Bijuu as well as the most difficult to control.The Ten-Tails is the original Bijuu and it's charka was spilted into the Nine tailed beasts.
In the world of Naruto there exist tailed beasts. These are called Bijuu, they have been around since before the founding fathers of the Hidden Villages. Some speculate as late as the first sennin Rikudou Sennin.
Since ancient times especially during times of war, and strife countries and clans have sought after their immense power. A single Bijuu is said to contain enough chakra to destroy an entire village or country if properly managed. Bijuu are unique not only in the fact that they can house so much chakra but the nature of their chakra itself is said to be unique with no equal. The Kyubi's chakra has been described as sinister with only Uchiha Madara possessing power sinister chakra than itself.
Each Bijuu possesses different attributes towards their chakra and how they use it, making an already difficult battle with a Bijuu even harder as predicting its power based on facing another of its kind doesn't apply to them. Bijuu are also different in the fact that unlike other animals of great size they are too powerful to tame via contract. Therefore very few ninja were capable of using them in combat so far the only ones known to have the ability to "tame" their chakra has been Uchiha Madara with the power of the fully awakened Mangekyou Sharingan, and the Shodai Hokage which possessed a special genetic trait to be able to tame all types of Bijuu.
Even so countries have tried desperately to tame these beasts of power. It wasn't only until an advanced ninjutsu technique as developed to "cage" a Bijuu's essence within a selected ninja. Thus creating a (controllable atomic bomb) so to speak. But this technique required sacrifice of human life, and even then only a certain percentage of a Bijuu's power was controlled. This S rank technique gave birth to Jinchuuriki which translates literally as "power of human sacrifice". But even with each nation possessing their "precious" jinchuuriki the Bijuu's power would not be tamed without consequence on the host. Often times jinchuuriki are hated because they the host are seen as the demon themselves and not the container of it. Other times Jinchuuriki are considered simply weapons of war whose sole purpose is to protect their village, this leading to the erosion of the host having a personal life outside their duties. Later in the Naruto series it is revealed that the Shodai Hokage gathered 8 of the Bijuu using his special genetic trait and split them between countries to maintain power and cease a great deal of the war.
In Naruto Volume 50, Chapter 467, it was revealed that the 9 bijuu are the nine divided parts of the Juubi's chakra, if Akatsuki capture all nine the Juubi will be reborn and Madara will be able to complete the MoonEye Plan.