An owner of a particular song is the songwriter, however the publisher will usually negotiate a royalty split 50/50 of the publisher share. The songwriter is the copyright owner who composes the music and/or writes the lyrics. If somebody writes the lyrics and someone else composes the music, then the pie is split even further between the two. Add a few performers like Britney and the Band and the pie gets even more complicated, and usually its the composers/songwriters who get squeezed down to their writer's share.
A publisher is like a record label, the 800 pound gorillas like EMI or SONY/BMG or a guy like me who purchases hosting and a domain name out of my own pocket to upload my own music. Publishers include everyone and everything that handles administration, marketing, and A&R - like the producer. Remember how a $250,000 advance gives the recording artist a $4,000 net income from a semi-successful record? This is due to the publisher's power. They sign the artists with bunk deals that figure in inflated estimates. If the album flops, the publisher takes their cut which is before the artist's. Successful artists with a good track record can maybe squeeze the publisher out of 50% of the publisher's share. New artists with no track record could have their number cut in half in favor of the publisher, who is racking up the overhead to record the artist, market, and replicate the artist's estimated number of albums for sale. Out of all royalties paid by the media users, the pie is split up, but first collected by the PRO (performing rights organization). The PRO distributes the shares 50/50 between Publisher(s) and Songwriter(s), which are then distributed based on the deals negotiated.
Aren't you glad you are an unsigned, independent musician? Seems like everybody's out to get you to take advantage of your sweet nectar. No worries, just always exude caution before signing up for one of those "deals" we all have heard about. If something seems too good to be true, it most often is not true. I've always wondered what it would be like, though to actually be signed with a major 1200 pound gorilla and be working with them in their hierarchy. Would it really be like Tracy Morgan in 30 Rock?