Sharingan,which also means Copy wheel eye, is one of the three Great Dojutsu. Other two being Byakugan and Rinnegan. Sharingans basic ability is to see the chakra flow when activated. The eye's power gives a colour to chakra allowing the wielder to differentiate them. Because of this,the user also has the ablility to see if someone is under a genjutsu because then the chakra flow of the observed person would be irregular. The second ability grants the user an incradible clearity of perception thus giving them the power to recoginze genjutsu and diffrent chakraforms or te ability to predict your opponent's movement,and thus counter them. This ability also allows the user to pick up subtle details enabeling them to read lip movements and/or mimic something like pencilmovements. As your sharingan gains more tomoes this ability improves untill it gives the wielder such clarity that he/she can predict enemies attacks, allowing them to see the opponent's next move from the slightest muscle tension in their body. But meanwhile seeing the attack doesn't assure the user that he will definetly avoid it. Sharingans last ability is to copy almost any technique that it sees (as long as its not a kekkei genkai or summons that requires a contract)
The final ability is that the sharingan can put someone under hypnosis simply if the person looks the wielder in the eye(s). More advanced and experienced sharingan wielders can even hypnotise Summonings like Manda or even a tailed beast and controll them.
*Sharingan stages* :
*1 tomoe* : allows the user to see chakra flow and see through genjutsu up to D rank.
*2 tomoe* : allows the user to see chakra flow and see through genjutsu up to C rank.
*3 tomoe* : allows the user to see chakra flow and see through genjutsu up to B rank(i think) and to Copy techniques.
*Mangekyo Sharingan* : Is a powerfull sharingan that gives the wielder the ability to use Susanno and other techniques like Tsukiyomi, Amaterasu or Kamui. But this stage has a mayor flaw,by overusing it the user can loose his sight.
*Enternal Mangekyo Sharingan* : EMS is basically the same as MS,but the user isn't exposed to the danger of loosing the eye sight anymore.