A short blurb from yet another article that discusses the changing definitions of songwriting and hooks (http://www.salon.com/ent/music/feature/2000/05/10/pop_song/):

....The traditional definition of a song -- a lyric set to melody over chord changes -- works fine for Jewel, Bob Dylan or Celine Dion. But it doesn't work for Metallica, Public Enemy, the Wu-Tang Clan or any other groups that build songs from riffs, beats and sound effects. Today's songs are fluid enough to embrace elements besides lyric and vocal melody. ...a defining hook in a hip-hop song is its catchy bass line, or a metal song's hook is its snarling guitar riff...