Timmy is a character in South Park. He's a little boy that's retarded and bound to a wheelchair, most probably paraplegic. He can't say much else than his name, maybe a few easier phrases.
His band is called "Timmy and the Lords of the Underworld". Their first song is called like that, too. The first line of lyrics was that phrase, too. Can you notice a pattern? :-) The second line of the song was "Darkness fills my heart with pain", but Timmy doesn't sing that, Skyler and the Lords do. The Lords play some fast rock, Timmy yells his name all the time, and that's about it. Amusingly enough, they become more popular than Phil Collins in one episode.
They had another song, it was called "Livin' A Lie".
It's a bit different than the other one, there's some pseudo-rap in it, but it's also pretty much pointless and is based on Timmy shouting his name in various ways.
[after hearing Timmy's song]
Cartman: That was awesome!
Stan: Yeah, Timmy rules!
Timmy: Timmy!