The Uchiha clan are shinobi born into the Kingdom of Ninja Town. Most commonly they originated from Konohagakure, or present-day Yamigakure. With two different timelines of the clan's power, it's hard to say which history belongs to either.
One half of the clan tells a story of a god that cursed the family's great power and successes. This curse was manifested in a seal that was placed on every member's back that was in the shape of a circle and a line. Which lated evolved into the Clan's emblem. This curse's power came to power when the clan elder died, of which his children's eyes cried for several days and evolved into Sharinga during the process. The power was strong, and with excessive use, the user was rendered blind. One brother later gave one of his eyes to his brother to grant him sight, and with it, a greater power.
The other half say Uchiha are descendants of Indra, the elder son of Hagoromo. Through Indra, the Uchiha inherited Hagoromo's "eyes", granting them powerful spiritual energy and chakra. As part of the so-called "Curse of Hatred" first displayed by Indra, Uchiha are subject to powerful emotions: these emotions typically start as love for a friend or family member that then become overpowering hatred when the object of their love is lost. This loss, whether actually experienced or merely threatened, awakens an Uchiha's Sharingan.