X Factor is a reality singing competition created by Simon Cowell and it’s kinda similar with American Idol except for the following: (1) the competition is open to both solo artists and groups, (2) has no upper age limit, and that (3) the judges act as mentors to the final contestants.
The reason why he left American Idol is because he is bringing X Factor to the US. The show is set to premiere on September this year and Fox has aired its first teaser trailer during the Superbowl episode of Glee.
The trailer features Justin Bieber, Katty Perry, The Pussycat Dolls, The Blackeyed Peas, Usher and Madonna.
Check it out here: