Sharingan

The Sharingan (literally "Copy Wheel Eye," English manga "Mirror Wheel Eye") is a special type of pupil condition that naturally occurs in all members of the Uchiha clan, though not all of them gain the ability to use it. Itachi hints at some dark ultimate purpose behind the Sharingan, which both he and Sasuke apparently know, but the series has yet to reveal exactly what that purpose is. The Sharingan is thought to be descended from the Byakugan, but grants the user different abilities and a different insight.

The Sharingan's first and most well known ability is to memorize any technique that it witnesses. It can memorize ninjutsu, genjutsu, and taijutsu with perfect accuracy, allowing the user to use the techniques as his own. In order to reproduce copied jutsu, however, one must have the necessary skill or ability to perform them. In addition to physical skill, the Sharingan cannot allow the user to reproduce other kekkei genkai (blood inheritance limit, bloodline limit) since the Sharingan user doesn't possess the same genetic trait. Additionally, Sharingan users cannot summon a creature which they have not made a contract with.

The Sharingan also grants the user incredible clarity of perception, allowing them to track fast-moving objects with ease and even predict the subsequent movements of those objects. The ability to track and predict movement is largely based on individual skill, as unskilled users have much more trouble tracking fast-moving objects. A fully-evolved Sharingan eye can track all even the quickest opponents, even if the user can't move at the same speed. This perception is also great enough to see through genjutsu and to observe the movement and flow of chakra, but not to the same extent or clarity as the Byakugan. Additionally, the Sharingan grants the user exceptional analytical powers, allowing them to instinctively pick up on various visual clues with little difficulty.

Finally, the Sharingan is capable of a unique brand of hypnosis that involves suggesting actions and thoughts to the opponent. When used in conjunction with the aforementioned aspects of the Sharingan, this allows the user to perfectly mimic every move their opponent makes, even before they do, making it appear as if the user can see the future. It also allows for other forms of hypnotism, such as putting an opponent to sleep, relaying memories to someone else, or creating various illusions to fool an opponent.

The Sharingan does not automatically appear from birth in a user; instead, it will usually appear when the user is in a life-threatening situation, where the power of the Sharingan could save the user. After this first appearance, the user can then call up the Sharingan any time he or she pleases. The power of the Sharingan is judged through the number of tomoe in the eye, known as the "tomoe seal". It is not unheard of for a Sharingan user to have different amounts of tomoe in each eye. For example, Sasuke's Sharingan initially had two tomoe in one eye and one in the other. Upon his first regular activation of it, it had evolved to two tomoe in each eye. He evolved it to three in each eye during his battle with Naruto at the Valley of the End. A fully-matured Sharingan has three tomoe in each eye.

The Mangekyo Sharingan (literally "Kaleidoscope Copy Wheel Eye") is a heightened form of Sharingan. The Mangekyo Sharingan is distinguished from a normal Sharingan through its appearance, which changes the form of the tomoe seal. However, unlike the Sharingan, it may have a different appearance between the users, as Kakashi's version is significantly different from Itachi's. 

The Mangekyo Sharingan was first obtained by Uchiha Madara and his brother. In order to obtain it, Madara killed his closest friend and awaken the Mangekyo. However, as the use of Mangekyo prolongs, the eye begins it descent into darkness, rendering the user unable to perceive light which almost the same as blindness.

As Madara's eye begins to turn blind, Madara starts to lose his sanity. Ultimately, he killed his own btother and stole his eyes. After planting his brother's eyes, the Mangekyo Sharingan was made permanent and able to perceive light again. This was the curse that Itachi referred to when talking about the Uchiha clan to Sasuke. The curse says, an Uchiha must kill his closest frient to awaken the Mangekyo Sharingan, and kill his sibling to make it permanent.

Mangekyo Sharingan is a powerful eye technique. It has the most powerful form of genjutsu, where an opponent can be trapped just by eye contact. One such genjutsu - Tsukiyomi, traps an opponent in another dimension where days feels like years and they are subjected to torture. Mangekyo Sharingan is also capable of hypnosis, where Madara managed to control the Kyuubi to attack Konoha.