In the 火影忍者 shippuden episodes 119 and 120, they explain everything. i really suggest 你 watch them. obito has become one of my 最喜爱的 characters, so NO SPOILERS!
When he was young and during ninja world war 3 (if i'm not mistaking) he got cut and 迷失 his left eye. when his team mate Obito Uchiha was smashed 由 a rock, their other team mate Rin transplanted Obitos sharingan into 卡卡西 :) it's a really sad story... :'( (this is my own drawing) :D
During the Third Shinobi World War, he was sent on a mission with his team (Minato Namikaze, a medical-nin named Rin, and Obito Uchiha) to destroy a bridge near Iwagakure to prevent them from getting supplies.
During the mission, Rin was kidnapped. Obito advocated to go save her, but 卡卡西 wouldn't allow it. Obito ended up going alone, and 卡卡西 later swooped in to save him- losing his left eye in the process.
They ended up going to save Rin together (some sort of bond of friendship having been formed) and were able to rescue her.
But during the fight they damaged the cave they were in and it began to collapse. A rock fell on 最佳, 返回页首 of Obito, completely crushing the right side of his body. Knowing that he was going to die, he told Rin to remove his left eye and offered it (along with the Sharingan) to Kakashi. After much deliberating, 卡卡西 accepted it and fled the cave with Rin.
