The Sharingan is the Dōjutsu Kekkei Genkai of the Uchiha clan that appears selectively among its members. It is regarded as one of the "Three Great Dōjutsu", the others being the Byakugan and the Rinnegan. While its powers originated from Kaguya Ōtsutsuki's Rinne Sharingan, its independent form was first manifested by Indra Ōtsutsuki (Hagoromo Ōtsutsuki in the anime).
When a wielder of this kekkei genkai experiences a powerful emotional condition with regards to a person precious to them, their brain releases a special form of chakra that affects the optic nerves, transforming the eyes into Sharingan; for that reason the Sharingan is described as an "eye that reflects the heart". Often, as per the Uchiha's so-called "Curse of Hatred", this emotion is a negative one, brought on by stress or loss. The emotion can also be positive, driven by a desire to protect or reunite with a loved one.
In the anime, it was shown that normally after first acquiring this power, the user's enhanced perception will throw off their timing, causing them to overstress their bodies from trying to keep up with the increased reaction time, forcing the user to adjust to properly move with it. It was also shown during emotional states that begin to trigger the Sharingan's awakening, it can temporarily blur the person's eyesight until finally manifesting. Assuming that the user is aware they have awakened the Sharingan they are there after able to freely use it. When first awakened, each Sharingan usually will have only one tomoe , although in Obito Uchiha's case, as well as Itachi and Indra's in the anime, they immediately gained two tomoe in each of their eyes. Through training and continued use, the Sharingan will develop a second tomoe and then, upon full maturation, a third. When Hagoromo awakened the Sharingan in the anime, he had all three tomoe immediately. All the Sharingan's abilities are available to the user from its earliest stage, but with greater development comes greater proficiency with those abilities.
As with any Dōjutsu, Sharingan can be transplanted into non-Uchiha. They are unable to deactivate it and its use has a greater chakra demand for them than for actual Uchiha, who need only minimal chakra to keep it active. For these reasons, non-Uchiha tend to cover their Sharingan when they aren't using it in order to conserve chakra. Sharingan that originate from the same Uchiha are still linked to some extent even when wielded by separate users, at times enabling those users to see the same things.