Stages
The Sharingan is not automatically activated in the user by birth. It is only first unlocked under some stressful or emotional condition after which the Sharingan may be used freely.
Abilities
The first of the Sharingan's powers is being able to see the flow of chakra.
The Sharingan's second-most prominent ability grants the user an incredible clarity of perception As the Sharingan evolves, gaining more tomoe seals, this ability extends to being able to track fast-moving objects before finally giving some amount of predictive capabilities to the user. However, even though the user may be able to see an attack or know it is coming, their body may not have time to react.
Receiving the Sharingan
The Sharingan needs to be activated by the user himself or herself under a difficult, emotional stress. After its first appearance, the user can use the Sharingan any time he or she pleases. It is also possible to transplant Sharingan from one person to another, as occurred with Kakashi Hatake, who received his left eye's Sharingan from his dying comrade, Obito Uchiha. Because of this.