The Ten-Tails is the original tailed beast and was sealed in the moon by the Sage of Six Paths.Today it exists as the group know as the 9 tailed beasts the form of it's monstrous charka.
As well as the ten tails common with its chakra-bijuu counterparts, it has one eye that has three layers of three-tomoe Sharingan in it.
In the Rikudo Sage's time, the Juubi was a monstrous rampaging beast, referred to as the 10-tails. To save the world the Rikudo Sage performed a jutsu that would seal it inside his own body, becoming the first ever Jinchuuriki. The Juubi would eventually be free once he died so he separated the Juubi's body from its chakra. The chakra would be split into 9 separate parts which would become the 9 bijuu that are known throughout the Naruto universe in the present day. He then performed another jutsu that would incase the body in a sphere of earth and sent it into the sky, where it would become the Naruto universe's moon.
After its chakra was separated, the 9 bijuu were scattered across the world, over time the first eight were brought under the control of the 1st Hokage, who in the spirit of keeping tensions low between the five main ninja-villages, allocated bijuu to each village. They in turn would use their own versions of the original soul sealing jutsus to create their Jinchuurikis.
Madara Uchiha would then take control of the Kyuubi and fought the 1st Hokage, but still lost and was presumed dead.
Tobi's plan, in which he has had Akatsuki capturing Jinchuuriki's and extracting their bijuu for years, will become complete with the capture of the Hachibi (Killer Bee) and Kyuubi ( Naruto Uzumaki). Once he has all nine, he will be able to restore the Juubi and become the Juubi-Jinchuuriki himself, increasing his already omni-powerful Eternal Mangekyou Sharingan. This will allow him to create the most powerful genjustu, "Infinite Tsukuyomi" (Mugen Tsukuyomi), which he will place on the moon itself. The jutsu will reflect on the entire planet, making everyone one with himself.
To stop this, the five great ninja villages and the Iron Country, headed by the current Raikage, have formed the first ever Shinobi Alliance to participate in the fourth great ninja war, specifically for the purpose of fighting over Killer bee and Naruto against the remaining members of Akatsuki and the seven bijuu Madara has under his command.