What is a music publisher?

A music publisher used to publish sheet music. When the income from that source decreased, he became a promoter of songwriters and composers. He tries to achieve that covers are being made and that songs are being used as (background)music in television programs and movies. He also takes care of monitoring the use of compositions and collecting royalties. In return, the music publisher receives a percentage of the royalties, which can be as high as 1/3 (in some countries even more).