Sharingan

The is a Kekkei Genkai, which appears in some members of the Uchiha clan. Some think that it was derived from the Byakugan. Although it consumes chakra while active, the chakra levels that Uchiha must exert is so insignificant that they can keep it active near-constantly.

Stages
When the Sharingan is first activated by a user, it has a different number of tomoe around the central pupil, varying between users. The wielder of the Sharingan, through continued use, evolves the Sharingan which involves gaining tomoe. As the Sharingan evolves the user gains new abilities until the maximum of three tomoe is achieved. The only level up from there is the Mangekyo Sharingan, which is gained by the loss of someone close to the user.

Abilities
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. A prime example is Rock Lee's brand of taijutsu. While Sasuke was able to copy some of Lee's moves, they put a much more significant strain on him since he hasn't trained nearly as much as Lee has. In addition to physical skill, the Sharingan cannot reproduce jutsu dependent on other factors, such as other other kekkei genkai or summons that the user has not signed a contract with.

The Sharingan's second most prominent ability grants the user incredible clarity of perception, allowing them to easily recognize genjutsu and different forms of chakra, though not to the same extent as the Byakugan. This also allows them to pick up on subtle details, enabling them to read lips or mimic something like pencil movements. As the Sharingan evolves this ability extends to being able to track fast moving objects before finally giving some amount of predictive capabilities to the user. Even though the user may be able to see an attack or know it is coming, they may not have time to react.

The Sharingan's final ability is capable of a unique brand of hypnosis that involves suggesting actions and thoughts to the opponent through simple eye contact. When used in conjunction with the other 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. More advanced Sharingan users can take the hypnosis ability even further by tricking or even trapping their opponent within an illusion. The potency of the Sharingan's illusion is dependent on the user's knowledge and skill with implementing Genjutsu.

Receiving the Sharingan
Unlike the Byakugan, the Sharingan does not automatically appear from birth in a user; instead, the user has to awaken it. After this first appearance, the user can then call up the Sharingan any time he or she pleases. It is possible to transplant Sharingan eyes from one person to another, as occurred with Kakashi and his deceased comrade Obito Uchiha. Kakashi's Sharingan cannot be deactivated. Because of this, and because using the Sharingan requires much more chakra from him than it does an Uchiha, he usually covers it to conserve chakra.

Transplanting the Sharingan eyes from a living or deceased Uchiha is the only way to avoid the loss of sight over time from the Mangekyo Sharingan. The user is then said to have an 'Eternal Mangekyo Sharingan'.