Sharingan is one of the 3 dōjutsu and a kekkei genkai, know only to the uchiha though only strong members of the uchiha may acquire these eyes, the Sharingan can only be awakened by strong emotions this causes the optic nerves to alter and turn the eyes red as explained by the second hokage in the manga/anime. there are 5 stages of the Sharingan there is the 1tomoe Sharingan, 2 tomoe Sharingan, 3t tomoe Sharingan, mankeyko Sharingan, then finally eternal mankeyko Sharingan this is all the stages of the Sharingan
the 1 tomoe being the first stage as the eyes are activated, after the user experiences a life threatening situation, once awakened the user becomes able to pierce through D Ranked vision genjustu, and would be able to follow the movement of most taijutsu moves as well ninjustu techniques, also it can see the flow of charka and whether it is regular or irregular flow, this is how they tell there in a genjustu.
the 2 tomoe Requires one to have already awakened the first tomoe once the 2nd tomoe is gained the user becomes able to perceive motions as well as copy them, one becomes able to now pierce through C rank visual genjustu and is able to track speeds almost double that of the users, with the 2nd tomoe the user also becomes able to predict to a certain extent the use of ninjustu related techniques through reading handseals, then able to make out what justu that opponent might use.
the 3 tomoe Requires the 2nd tomoe once awakened the user not only becomes able to perceive motions and predict them, but also copy motions perceived shortly before they happen, the user becomes able to track even more higher speeds than with the 2 tomoe, the user would be able to copy hand seals performed by the opponent as long as he is able to perform the technique being copied, and with this Sharingan, the user is able to pierce through B Ranked visual genjustu, also the user can remember all the justu he has copied and be able to use them in future battles,
the mangekyo Sharingan The Mangekyo Sharingan is awakened through the trauma experienced from the death of a person very close to the Sharingan user. With the user normally having to witness or experience this first hand this causes the pupils in the eyes to change pattern to form a new shape, Using the Mangekyo Sharingan puts strain on the user's eyes and erodes their vision, eventually causing blindness. This can be prevented by transplanting another pair of Mangekyo Sharingan from a close relative, such as a sibling which make the eternal Mangekyo Sharingan
the eternal Mangekyo Sharingan, stops the Mangekyo Sharingan from going blind and obtain a new power but it is impossible to say as all Mangekyo sharingan powers are different as each set of eyes have there own set of power
how was this korra sensei?