Bijuu are demons that exist in the world of Naruto, they are also known in as demons in Japanese mythology. There are nine in total. 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, so far there are only two known Jinchuriki and they are Gaara and Naruto. Every demon has a different amount of tails, ranging from one tail to nine.

Seal used on Naruto

Kyuubi(Nine Tailed Demon Fox)
the nine tailed fox
Hidden Village:Leaf
Classification:Demon
Tail Number:kyuu(nine)
Type:Fox
Imbued Within:Uzumaki Naruto
Info:12 years ago the nine tailed fox appeard.Everytime it shook its tail it would
cause tsunamies and strong winds.Then a great hero of the hidden leaf village
sealed the nine tailed fox into a young boy name naruto.The hero was the forth
hokage.A group called the akatsuki search for this 13 year old boy naruto to take
the nine tailed fox that is sealed inside him.When sasuke was heading to orochimaru
naruto had a fight with him.He used his nine tailed fox chakra to stop sasuke.But at the
end of the fight sasuke won and naruto failed to bring sasuke back home.

shukaku(One-Tailed Demon Tanuki)
the shukaku
Hidden Village:Sand
Classification:Demon
Tail Number:Ichi(one)
Type:Tanuki(raccoon dog)
Imbued Within:Gaara
Info:Shukaku is one of the "bijuu",a tailed beast of the
ancient world.Seeking a way to create a ninja with a large
amount of chakra that would be an unstoppable machine,
the kazekage imbued Shukaku within his unborn son,Gaara.

Nibi(Two-Tailed Demon Cat)
Nibi Two-Tailed Cat
Hidden Village:Cloud
Classification:demon
Tail Number:Ni(Two)
Type:Cat
Imbued Within:Nii Yugito
Info:The two-tailed cat is imbued within Clould kunoichi Nii Yugito.
The nibi appears to have the ability to emit balls of spectral-
like fire.Highly destructive,the Nibi is also known as an
"ikiryou" or living ghost.

Sanbi(Three-Tailed Demon Turtle)
sanbi Three-Tailed Turtle
Hidden Village:unknown
Classification:Demon
Tail Number:San(three)
Type:Turtle
Imbued Within: ---
Info:The three-tailed turtle is imbued within no one.The
sanbi lives in an ocean area and has remained
uncaptured by any ninja until Akatsuki arrived.

Yonbi(Four-Tailed Demon)
Yonbi Four-tailed Demon
Hidden Village:Unknown
Classification:Demon
Tail Number:Yon(four)
Type:?
Imbued Within:?
Info:The four-tailed demon was imbued within an elder Jinchuuriki.
It is unknown what animal the bijuu was.The large black mark on
the Jinchuuriki's nose and cheeks may be a influence of the bijuu.
The Jinchuuriki used various elemental fusion techniques,a
probable by-product of the demon's power.

Hachibi(Eight-Tailed Demon Snake)
Hachibi Eight-Tailed demon Snake
Hidden Village:Unknown
Classification:Demon
Tail Number:Hachi(eight)
Type:Snake
Imbued Within:?
Info:The eight-tailed demon snake is presently free
and at large.It is unknown if it is sealed within a
Jinchuuriki.

Gobi(Five-Tailed demon Wolf)
Gobi Five-Tailed Demon Wolf
Hidden Village:Unknown
Classification:demon
Tail Number:go(five)
Type:Wolf
Imbued Within:?
Info:Gobi no Houkou, takes the form
of a dog in Japanese mythology.
This demon has not featured in the
Naruto series as of yet.

Rokubi(Six-Tailed Demon Ferret)
Rokubi Six-Tailed Demon Weasel
Hidden Village:Unknown
Classification:Demon
Tail Number:Roku(six)
Type:Weasel
Imbued Within:?
Info: Rokubi no Raijuu, takes the form of
weasel in Japanese mythology. This
demon has not featured in the Naruto
series as of yet.

Shichibi(Seven-Tailed Demon)
Shichibi Seven-Tailed Demon
Hidden Village:Unknown
Classification:Demon
Tail Number:Shichi(seven)
Type:badger
Imbued Within:?
Info:Shichibi no Kaku, takes the form
of a badger in Japanese mythology.
This demon has not featured in the
Naruto series as of yet.