doujutsu is the eye ability that doen’t required to use hand seals to cast spell. The subset of Doujutsu is rinnegan, sharingan and Byakugan. Doujutsu can be cast by looking to the eye of the opponent. Since they are eye jutsu user, the only way to fight doujutsu user is to fight them from their back since every doujutsu user have at least one blind spot that they cannot read or sense, except for rinnegan the weakness is still unknown. The sharingan user has special ability to stop doujutsu spell, then copy opponent spell and use it back against them. Currently the only known sharingan user alive of Uchiha clan is Sasuke, Itachi, Madara and Kakashi Hatake is a special case since his is not Uchiha but have Obito sharingan eye. Byakugan is the ability that belong to Hyuga clan have the ability to see 360 degree and opponent chakra flow, they also have the ability to cut the chakra flow of victim making them unable to use chakra temporary. Doujutsu is surely the most powerful jutsu in Naruto.
For more information on Rinnegan Doujutsu in detail.
For more information on Sharingan Doujutsu in detail.