The sharingan is one of the three legendary Dojutsu in the Naruto world, the Sharingan is a Red eye Kekkei Genkai that belongs to the Uchiha clan, for Uchiha clan members, in order to awaken the Sharingan they need to pass through huge pain and a really painful situationn and even then they can only awaken the Sharingan with one Tomoe which allows them to see the opponent's chakra flow, copy the enemy's Ninjutsu, and see through ordinary and weak Genjutsu, then, in order to awaken the second and third tomoes, they must take training sessions and train hard.
The complete Sharingan is a very strong Kekkei Genkai that can be used to catch up fats opponents, see through strong Genjutsu, and cast strong Genjutsu that can be fatal in battles. Anyway using Genjutsu against Sharingan users in battle is useless and looking them in the eye can be fatal, so fighting them is quite dangerous, and it can more dangerous when the opponent fights someone who has unlocked the Mangekyō Sharingan or the Eternal Mangekyō Sharingan, these two Evolutions of the SHaringan can extremely increase the power of the Visual Genjutsu and unlock another Keggei Kenkai (Blaze Release) which gives the ability to use some strong and forbidden techniques that can be used for combats and not illusion.
Each Uchiha member is blessed with a strong ability they can use after unlocking the MS/EMS, and none else can these techniques (Obito's Kamui, Shisui's Kotoamatsukami...)
After mastering the EMS, the talented Shinobi with strong skills and chakra controlling may unlock the Rinnegan after acquiring the Senju clan cells.