Genjutsu: Sharingan broadly refers to any genjutsu performed with the Sharingan. It has also been shown performed through the Sharingan's higher evolutions, the Mangekyō Sharingan and the Rinnegan, although it's not always clear what difference this makes, if any.
By establishing eye contact with a target, the Sharingan user locks them within a genjutsu for one of various purposes: instantaneous, but temporary, unconsciousness in the target; forceful extraction of any information the target possesses; relay memories to the target; remove genjutsu placed on the target by others; controlling a target's actions. Members of the Uchiha clan are particularly famous for using their Sharingan to control the Nine-Tails, such that Konoha quickly came to suspect an Uchiha's involvement in the Nine-Tails' Attack on Konoha.Sasuke Uchiha is noted to be able to use his Rinnegan to trap tailed beasts in a genjutsu with a mere glance.
The illusions created by Genjutsu: Sharingan do not appear to have any trademark form, appearing wildly different between the same functions and even the same users. For example, Sasuke typically uses it to render targets unconscious, yet on the first occasion the target's body is pierced with large feathers, on the second the target is grasped and overwhelmed by a shadowy figure, and on the third the target experiences Sasuke personally stabbing them.
A shadowy version of Sasuke's humanoid Susanoo first appears in a genjutsu he creates, only making a formal début several chapters later.
In the anime, Madara reverts his Rinnegan to his Mangekyō Sharingan to use this technique.