If someone wants to use images, music, texts or film footage, he needs to have a permission for it. He can get this permission from the holder of the copyright. Basically the creator is the copyright owner. The copyright owner has the possibility to commission someone (a third party) to grant these rights to users against royalty payments. The third party usually is a publishing company, e.g. a music publisher. The advantage is that the author can concentrate on his artistry, whereas the publisher attends to its promotion.
Meanwhile it is possible for publishers to put a big amount of data online. Also for music publishers it is thus interesting to offer produced content online for listening and also to license it for certain application cases. In our Proud Music Library ou can acquire standardized licenses and use those usually unlimited in time or territory, if you for example want to use background music for a web site. Just have a look on the licensing models.