Bijuu are demons that exist in the world of Naruto, they are also known in as demons in Japanese mythology. Each one has an incredible amount of chakra within them, to try and combat these powerful beasts and stop the destruction they cause, they were captured and sealed inside new born babies using a powerful technique that would kill the person using it.

The Fourth Hokage was one of those people who sacrificed their life to seal a Bijuu and protect the world from chaos. When the human has a demon sealed inside them they are known as Jinchuriki. Every demon has a different amount of tails, ranging from one tail to ten.

The nine separate tailed beasts that exist today were created from the chakra of the the Ten-Tails, the original demon. The Ten-Tails was subdued by the Sage of the Six Paths and sealed within his body to become the first Jinchuriki. However, this method was only a temporary way of sealing the demon, as once the Sage died, the Ten-Tails would be free again. As the Sage was close to death, he used his "Creation of All Things" ability to split the monster into nine tailed beasts and scattered them across the world.

One-Tails
Ichibi no Shukaku, takes the form of a racoon. This demon was sealed within Gaara of the Desert, because of this, Gaara has the ability to control the sand around him. Other translations include: Shukaku, One tailed Shukaku, Tanuki.

Two-Tails
Nibi no Nekomata, takes the form of a two-tailed cat. This demon sealed within Yugito Ni.

Three-Tails
Sanbi no Kyodaigame, takes the form of a three-tailed turtle. This demon was previously sealed within Yagura.

Four-Tails
Yonbi no Saru, takes the form of a four-tailed gorilla. This demon is sealed within Rōshi.

Five-Tails
Gobi no Irukauma, takes the form of a five-tailed dolphin-horse. This demon is sealed within Han.

Six-Tails
Rokubi no Namekuji, takes the form of six-tailed slug. This demon is sealed within Utakata.

Seven-Tails
Nanabi no Kabutomushi, takes the form of a seven-tailed beetle. This demon is sealed within Fū.

Eight-Tails
Hachibi no Kyogyu, takes the form of a eight-tailed ushi-oni-like beast. This demon is sealed within Killer Bee.

Nine-Tails
Kyuubi no Yoko, takes the form of a fox. This demon is sealed within Uzumaki Naruto who has incredible stamina and can summon large amounts of the Kyuubi's chakra. However Naruto has not yet learned how to fully control the awesome power of Kyuubi. Other translations include: Kyuubi, Nine Tailed Demon Fox, Kitsune.

Ten-Tails
The Ten-Tails is the original demon, it was sealed inside the Sage of the Six Paths, until he split the demons chakra into the nine tailed beasts that exist today.