What are ASCAP and PRS?
Performing societies do you a favor (and also take their cut for administering the whole process) by collecting all music fees from public places. This includes venues, concerts, restaurants, radio stations, nightclubs, television stations, and large outfits of the same. Licensees (owners of all these public places that play music) pay the peforming rights organizations (PROs) a blanket license fee, which is based on what genre of music is being performed and how great an audience reach it has. The PROs then distribute or pay out royalties to publishers and songwriters throughout the year, depending on public airplay and broadcasting.
The prominence and length of your song is a tremendous factor in the amount you will be paid if your song ever makes it to broadcast TV. If you can get an intro song on a popular TV show on broadcast TV, you will see royalties for years to come! You will still receive money if your song is only played for a couple seconds in the middle...