Sharingan

Sharingan is one of the traits own by the Uchiha Clan, it gives the owner the ability to See chakra or chakra flow within a person. Also it can copy the movement of a person perfectly without having to memorize the movement. There are many function that the sharingan posses and one of them is to see through Genjutsu (Illusion Technique) or simply create one. 

One of the user of the Sharingan is Kakashi Hatake, who uses his sharingan in a different way and simply cannot use it all he want since he didn't belong to the Uchiha clan. Itachi claims that only an Uchiha can fully use the sharingan to its full potential.

The Sharingan, different then the Byakugan, is not automatically acquired by user at birth. It is unlocked through some stressful or emotional situation then which the Sharingan may become active and be used freely. One of the Sharingan's most prominent ability is to grants the user an incredible clarity of perception, which allowing them to recognize genjutsu and also different kind of chakra with an easy perception, though not to the same extent as the Byakugan. This ability also allows the holder of the traits to pick up on subtle details, allowing them to read lip movements or even mimic something like seal hand form and even Taijutsu movements. Sharingan does evolves by unlocking and gaining more tomoe seals, this ability extends to being able to track fast-moving objects before finally giving some amount of predictive capabilities to the user. However, even though the holder of the traits was able to see the attack or simply know where it is coming, their body when not trained may not have the ability or time to react at the correct moment.