US: Used to convey the idea of someone being cool or worthy, or that they have done something really amazing.
UK: Used by British people trying to bum lick others. Usually those trying to be ´Fake Americans´ and immitate characters from TV series such as Friends, Will and Grace or the Simpsons. Usually makes the user appear really stupid.
American Dude: ´Hey man, that show was awesome, you rock!´
British Boss (AKA Dickhead) ´That presentation was really good, you rock!´