A music publisher makes sure songwriters and composers get their money when their songs are used to make money for other people. The money they collect is called royalties.
Music publishers collect 6 types of royalties for songwriters/performers.
Synchronization royalties: paid by the producer of a movie/tv show/ad to the writer of the song for the right to “synchronize” the writer’s music to the producer’s moving images.
Special use royalties: Paid by creators of ring tones, karaoke, video games, etc. to the songwriter for the rights to use the songwriter’s work.
Print royalties: Paid when the song is sold as sheet music, lyric book, etc.
Performance royalties: (better understood as “broadcast royalties”) are paid by the Performance Rights Organizations (ASCAP/BMI/SESAC/SoundExchange/etc) to the writer of the song when the writer’s song is broadcast over public airwaves. This includes but is not limited to radio stations, satellite radio, tv stations, performance in live venues, online streaming services (Spotify/Rhapsody etc.), and YouTube. It’s impossible for radio stations and venues to pay artists for every stream, so they pay a blanket license to the PROs who then use some crazy formula to figure out how to pay artists.
Mechanical royalties: established by U.S. Gov. 1909 Copyright act: 9.1 cents is paid to artists for recordings sold by the label to the songwriter for the right to “mechanically reproduce” the writer’s song on CD or download, whether or not the writer is the performer of the song. Mechanical royalties are paid by interactive streaming services (Spotify/Rhapsody etc.) and sales in territories throughout the globe.
Sampling Royalties: If someone uses pieces of a song or remixes the song, both the owner of the master work and the songwriter/publisher are owed money.