Sharingan, Literally meaning copy eye wheel is one of the 3 great doujutsu besides the byakugan and the rinnegan. It is a Kekkei Genkai that is only given to the Uchihas. It is even called the heavens eye due to the powers it gives the user. The Sharingan abilities consist of 2 basic parts which are the Eye of insight and the eye of hypnotism.
The first ability consist of being able to see the chakra flow, and further more to differentiate the chakra from another one. The sharingan in able to give color to the different kinds of chakras so that he can recognize each type. They are even able to tell if a person is under the influence of a genjutsu, since the chakra flow would be disturbed. The 2nd ability consists of great perception, the sharingan gives the user the ability to read movements judging from the movements of each muscle, and even incredible clarity allowing them to see if they are trapped in a genjutsu or not. They even allow us to pick up subtle detail like lip movements which makes it quite easy to understand and make up the orders of the enemy. With the power of the sharingan growing and the number of tomoe increasing that power becomes even greater, allowing users to see through thick fog, see quite far, and predict every movement form the slightest muscle tension of the enemy.
The 3rd ability of the sharingan is the ability to copy nearly every jutsu the user sees with the exception of Kekkei Genkai. The user is able to memorize ninjutsu, genjutsu, and taijutsu close to perfect accuracy and is even able to use those moves always if the body of the user allows him to do so.
And the 4th and last ability of the sharingan is the ability to produce really strong genjutsu, which are very useful in battle. The slightest eye contact can put you into a genjutsu which is really hard to escape close to impossible if the power of the eyes is big enough. Yet another ability is if you master genjutsu really well and you get stronger the eye will be able to put giant creatures into genjutsu, allowing the user to control the beast. Very strong eyes can control even the bijuu....
The 1st stage of the sharingan is the one tomoe, which is activated from a 2nd training session. This sharingan will allow the user to track movements and sped a lot greater then they would actually be able to put through. It will alow the person to see chakra and be able to react at the best time possible against and attack.
The 2nd stage is the the 2 tomoe sharigan, which is again activated by a life death experience where yet again great hate is involved. After awakening this sharingan the user will have boosted the abilities of the 1 tomoe. He will be able to track faster movements and now even mimic them if the body allows him to do so. It will allow the user to see through the handseals and predict nearly every jutsu beforehand. And he will be able to pierce through C rank genjutsu and lower.
The 3rd stage is the 3 tomoe sharingan. At this point the user needs to go through the 3rd training session. The third sharingan will allow the user to copy and predict movements to near perfect accuracy. The user will be able to keep up with extremely high speeds, and read the predict the movements of the enemy from the slightest muscle flex. This gives you even the ability to see through B rank genjutsu and lower.
The "last" kind of sharingan which is the Mangekyo Sharingan. This kind of sharingan is only activated after experiencing the death of a close person and having that death affect you and galvanize you. This eye is feared to be the strongest Kekkei Genkai in Naruto world. This stage of the sharingan gives the user a unique pattern which is different for every user. This stage allows you to read even faster movements, copy them even quicker and react to them in seconds. Despite that you can be able t see through A rank Genjutsu with this and, if you have mastered the genjutsu you can cast one with just eye contact without the need for seals. The negative this is that this sharingan can only be activates 2 times per battle and it consumes a lot of chakra and with the overuse the sharingan will become blind and it can't be healed until the end of the match. Something unique is that the user can be able to perform very deadly hidden jutsu which differentiate form user to user. But using those will cause the rundown time of the sharingan to decrease.
The Eternal Mangekyo Sharingan is a derivation of the Mangekyo sharingan and can only be obtained by taking the eyes of the siblings. The Eternal Mangekyo is the highest form of the sharingan known and it is feared to be the strongest of all doujutsu. This eye will give the user even further greater abilities then the EMS and able to cast all genjutsu (if he mastered them) by only looking. This sharingan can access even the forbidden techniques of the previous MS and unlock new ones. And unlike the MS the user won't have to suffer the damage from the normal MS, giving him the ability to use the MS without any side effects.