Music publishing starts with ownership. Copyright protects an original, tangible work and bestows exclusive rights to duplicate, perform and distribute the work upon registration. Technically, once you have copyright, you are the publisher.
Many songwriters choose to work with publishers. Publishers pay for copyrights and in turn administer by finding users, issuing licenses, collect income and distribute royalties. This relationship is established via the "publishing deal" which is an agreement between the songwriter (the owner of the copyright) and the music publisher (who seeks to own or rent the songs).
The most popular kind of publishing deal signed is a deal where the publisher's share of the royalty income is an industry-standard half. This is known as the "publisher's share" while the songwriter retains the other half, the "writer's share." Some other issues covered in most publishing agreements are duration, territories, advances, royalties, ownership and costs.
Music publishing pays. A royalty is the income derived from the song and is paid based on shares--the portion of ownership---dictated by the publishing agreement. Many publishers will also issue a writer an advance--an amount paid upfront on a publishing contract--and recoup the amount.
Music publishers vary in size. The largest and most prevalent are known as the majors. They are usually affiliated with record companies and movie studios like Warner, Universal and Sony/ATV. Alternatives to the majors come in the shape of independents ("indies") and writers/musicians that handle their own publishing.