A Performance Right Organization (PRO for short) exist to work on the behalf of musicians, ensuring they receive a fair share of public performance royalties. PROs work with broadcasters and distributors of media and track presentations. From that data, they bill the distributors a percentage of their revenue and forward the performance royalties to the artists.
Without the PROs, musicians would never get paid their fair share of public performance royalties. We highly recommend you become a member of a PRO in your country. In North America, the main PROs are ASCAP, BMI, and SOCAN.
There is a good comprehensive list of PROs on Wikipedia.