Shared copyrights

If you write the lyrics to a song and your buddy writes the music, then you each own 50% of the song. You don't own all of the lyrics and your buddy doesn't own all of the music -- you each own 50% of the total song, music, lyrics and all. This means you can't give someone exclusive rights to the song on your own if you have a fight with your buddy. And, if you make any money on the song, half of that money must go to your partner.

Other forms of shared copyrights come into play when you or your publisher (typically you give control of the song's copyright to the publisher) sign over a portion of the copyright to another publisher for a sampled composition -- a song that uses a portion of another song.