This is a song by American rapper Eminem, released as the lead single from his third studio album, The Marshall Mathers LP. The song is an ode to Eminem's alter ego, Slim Shady, and is a humorous take on the rapper's life and career. The song
From Wikipedia
"The Real Slim Shady" is a song by American rapper Eminem from his third studio album The Marshall Mathers LP (2000). It was released as the lead single a month before the album's release.