Lengthier answers can be found HERE: http://www.taxi.com/faq/publishing/#q1

Question:
What is publishing?
Answer:
Publishing rights are the rights to a song. If you write a song by yourself, you own the publishing (and copyright) from the moment you finish the song. You don't have to set up shop as a music publisher to own those rights - they automatically come with authorship.

Question:
Do I need to start my own publishing company?
Answer:
Typically not. You already own all of your publishing rights from the song's inception. People typically start a publishing company when they get a song "cut" and need a mechanism or company to which they can have the income flow.

Question:
How do I make money from publishing?
Answer:
A song generates money for the writer(s) when it appears on an album, gets played on the radio, used in a TV show or movie, gets sold as sheet music, and even when it is used as a ring tone on a cell phone. When the song is part of an album or is sold as sheet music, the writer gets a mechanical royalty. When a song is used in a movie or TV show, a performance royalty is paid to the writer(s).