Well, the Sharingan is obviously one of the great 3 doujutsus.
The sharingan belongs to the Uchiha clan, though there are only 2 remaining users of this doujutsu left. One being Madara Uchiha, the founder of the clan, and 1 being Sasuke Uchiha, the only person in the clan that wasn't killed by his brother Itachi.
The Sharingan awakens after a serious incident or very tiring exercise. Like a near death situation (most of the time this is the case), unlike the Byakugan or Rinnegan.
The Sharingan has 5 stages.
1 tomoe
2 tomoe
3 tomoe
Mangekyou Sharingan
Eternal Mangekyou Sharingan.
Each phase enhance the users abilities in to use their sharingan. MS gives new abilities like Amaterasu, Tsukuyomi and Susannoo. EMS is still unclear.
It gives you the ability to use and withstand certain Genjutsus, increases the visual powers in Taijutsu, making you react faster, in Ninjutsu, by being able to react faster (seeing hand seals, chakra's etc) and also allows you to copy anything you are capable of copying (no KG's + you need the affinity for it.)