PRO = Performance Rights Organization. PRO handles your publishing and writers share of royalties. The Master or Mechanical side of your composition will be handled by your label, or In most cases, agency's like Harry Fox, or in the UK the Mechanical-Copyright Protection Society, MCPS (now in alliance with PRS), acts to collect (and distribute) royalties to composers, songwriters and publishers for CDs etc...all of whom distribute blanket license agreements to end users.
PRO is usually a not for profit government controlled org. ie PRS (UK), BMI (US) and ASCAP (US), but some like SESAC (US) are independently operated. PRS= Performance Rights Society. PRS is the leading UK PRO. Nearly every country has their own PRO royalty enforcement agency to ensure its musical citizens are paid for their works. Your PRO will work with other PRO's in other countries to ensure you are paid appropriately for any synchronization licensing that has occurred over seas. For example, if your music was licensed in a US TV show, PRS will work with ASCAP to make sure you are paid for US public performance (TV airings and repeats, radio etc). If you hear it, but can't take it with you, that is public performance.
In most cases when you license a song for a placement in a film or TV show, they will require the Sync use agreement and Master use agreement. The Syncronization agreement is for your publishing and public performance rights. The Master use agreement is for the mechanical physical rights for use if the production goes to DVD or they choose to make a CD soundtrack etc. The Sync and Master rights use is usually split 50/50. 50% to the publisher (You if not signed) and 50% to the record label (you if not signed).
When registering with a PRO, you should register as writer and publisher if you control your own publishing. You will still receive 100% of all sync public performance royalties, but establishing your own publishing co will enable you to easily add songs with other writers involvement, and even retain a piece of publishing should you enter a publishing agreement with another publishing company in the future. You can sign other artists to your publishing if you are active in licensing and have relationships with your PRO or music supervisors.
If you are located in the UK and secure a placement in a TV show, you will likely require PRS to collect your publishing and MCPS to collect your mechanical royalties on your behalf.
Since I am in the US and more familiar with BMI and ASCAP and not familiar with PRS in the UK, you should do more research on their requirements for registration and reporting.
Hope that helps.
Mike Galaxy
www.BandPromote.com