Sharingan eyesis the Keikai Genkai of the Uchiha clan. Its literal meaning is Copy Wheel Eye. It is one of the three great dojutsus, others being Byakygan and Renegan.

　　As told by Tobirama Senju, the root of Uchiha powers are their love, greater than anyone anyone else's. But when they loose someone they love, their love is changed into hate. And this emotion of theirs is reflected by their eyes. Thats the reason it is called "the eye that reflects the heart".

　　It is activated when an Uchiha experiences any extreme emotion, such as pain, loss, etc, or when they really need it, the chakra in their brains takes a special form and affect their optic nerves which turns their eyes into sharingan.

　　When first manifested, there normally is one tomoe and with traning and use the number of tomoe increase, maximums till three tomoe. There are cases when someone directly manifests it with two tomoe, like Obito Uchiha, Madara Uchiha, etc. More the number of tomoe, better its abilities.

　　The basic abilities of the sharingan are -:

　　1) It allows the user to see the chakra of others. Not as good as the byakugan but still the user can see the basic flow of chakra.

　　2) The user can see objects movie at high speed. And the perception is so developed that the user can copy any ninjutsu, genjutsu or taijutsu, except the Keikai Genkai and clan specific jutsus, to near perfection.

　　3) The user can put anyone under a genjutsu with just mere eye contact. It a normally known as Genjutsu: Sharingan.

　　Mangotsfield Sharingan

　　Sharingan can also evolve into Mangekyo Sharingan, when someone with sharingan witnesses the death of a loved ones.

　　The look of the Mangekyo sharingan greatly diverse from the default sharingan, and is user specific, that is, its abilities and appearance differs from user to user.

　　Mangekyo Sharingan also gives its users some special abilities, which may vary from user to user. And anyone with both the Mangekyo sharingan can use susanoo. But excessive use of Mangekyo sharingan can weaken the user's sight, even to the point of blindness. It can be prevented by furnished evolved Mangekyo sharingan into Eternal Mangekyo Sharingan, this can be doen by transplaning a relative's sharingan (like a sibling).

　　The seperate abilities of Madara's Mangekyo are unknown, but he can activities susanoo. His susanoo is one of the strongest. It has a dark blue color and and has a samurai like look in its perfect form. It has 4 hands and wields 2 katanas.

　　Obito's ability is Kamui. Kamui is Obito's space-time ninjutsu. With his Mangekyo sharingan he can use long range Kamui (as shown by Kakashi) and his right mangekyo sharingan can be used for short range kamui and self teliportation. Obito's susanoo has a light blue colour and it wields kamui shuriken which can teleport anything it hits.

　　Shisui has the most powerful genjutsu ever, Koto-amatsukami. It is a genjutsu Shisui performs with his Mangekyo. It is so effective that the one under it dosen't even notices and is totally under the control of the caster. Shisui also has susanoo which is green in colour, and carries a drill blade.

　　Itachi's right mangekyo can use Amaterasu- the inextinguishable black flames, and his left mangekyo can perform Tsukoyomi which takes away the perception of time of the one unde it. Itachi's susanoo is redish-orange in colour and it has Toshuka blade, which can seal anyone for eternity, and Yata mirror, which makes any ninjutsu or taijutsu totally ineffective. Together these two makes Itachi invincible.

　　Like itachi, Sasuke's right mangekyo also allows him to use Amaterasu and ability of his left mangekyo is Inferno Control which allows him to control any type of fire. His susanoo is purple in colour and has a bow and arrow which abels him for long range attacks.