Traditionally, once a songwriter creates a song (or copyright) they grant the rights to a music publisher, who then manages one or more songwriters' catalogs, registers the songs with the US Copyright office, pitches the songs, issues licenses to those who wish to use the songs, then collects the royalties on the songwriters' behalf, and pays the songwriter. In the process, the publisher keeps a percentage of the income for their role.
The term "publishing", most simply, means the business of copyrights. A writer owns 100% of his or her copyrights and 100% of the related publishing rights until the writer signs those rights away by contract.
As defined by the copyright law, publishing has two separate meanings. First, it is the "distribution of copies of a work to the public by sale or other transfer of ownership, or by rental lease, or lending". Second, a work can be considered published if there has been an "offering to distribute copies of a work to a group of persons for purposes of further distributions, public performance or public display." In other words, publishing occurs when copies are printed and there has been a offering to distribute those copies.
As a practical matter, music publishing consists primarily of all administrative duties, exploitation of copyrights, and collection of monies generated from the exploitation of those copyrights. If a publisher takes on these responsibilities it "administers" the compositions. Administrative duties range from filing all the necessary registrations (i.e., copyright forms) to answering inquiries regarding the musical compositions.
One of the most important functions of both songwriters and music publishers is exploitation of a composition or "plugging" a song. Exploitation simply means seeking out different uses for musical compositions. Music publishers have professional quality demos prepared and send them to artists and producers to try to secure recordings. They also use these tapes to secure usage in the television, film and advertising industries.
Equally important as exploitation is the collection of monies earned by these musical usages. There are two primary sources of income for a music publisher: earnings that come from record sales (i.e., mechanical royalties) and revenues that come from broadcast performances (i.e., performance royalties).
Mechanical royalties are collected directly from the record companies and paid to the publisher. Performance royalties are collected by performing rights organizations - ASCAP, BMI, and SESAC - and then distributed proportionably to the publisher and directly to the songwriter.
In addition to plugging and administrative functions, it is also important to know that there is a creative side to music publishing. Since producing hit songs is in the best interest of both the writer and the publisher, good music publishers have whole departments devoted to helping writers growing and develop.
The creative staff finds and signs new writers, works with them to improve their songs, pairs them up creatively with co-writers and hopes the outcome will be hit records.
Administration, exploitation, collection of monies, and creative development are the major functions of a music publisher. Although a writer can be his own publisher (and, in fact, owns his or her own publishing rights until they are signed away), the larger publishers in the music business usually pay substantial advance payments to writers in order to induce them to sign a portion of their publishing rights to the publisher.
Keep in mind that, since every music publisher is different, it is important for the songwriter to assess both the business and the creative sides of a music publisher before signing a deal.