Every time your music is played on a radio station, in a bar, during a hockey game, or anywhere else publicly you are supposed to be compensated. This poses a major problem. How are you supposed to know who is playing your music & where you should send the invoices? On the other side of the coin, how is a bar owner supposed to know how to contact each band to be sure they are paid their 6¢, or whatever? Fortunately, there is a very simple solution.
Performing rights organizations exist to serve as the middleman. You have three choices, ASCAP, BMI, or SESAC. Once you have registered your music they will make sure you are paid every time your music is played. The next battle you have to fight is to make sure the people that are playing your music submit cue sheets that reflect that. Bar owners, music venues, and other places where music is played either sign up for a blanket license or pay based on ticket sales or traffic to their location. In theory, that money is supposed to make it back to you.
In addition to making sure your band is paid, performing rights organizations give special discounts to their members for hotels, health insurance, rental cars, and a ton of other places. Depending on which PRO you choose, some will really work with you to help develop your band. They have showcases at all the major music festivals and help a lot of bands find the right managers, labels, and agents.