Licensing is a subset of copyright law, as I understand it, but I am far from expert on this stuff. The composer who creates the composition originally holds a legal copyright to it, but often chooses to share the copyright with a music publisher. Copyright can be passed on in an inheritance or sold to a different party. People who want to arrange a composition, perform the piece publicly, or issue one or more types of recordings of a composition need to secure a license from the copyright holder to do so, $ecuring different licenses for different needs. What in recent history has been most problematic for DCI and other marching arts organizations is the synchronization license, but the fight recently seems to have expanded to include the reproduction right, the mechanical license, and the digital performance license as well.
http://www.bmi.com/licensing/entry/types_of_copyrights
Here are some basics on copyrights of musical compositions:
http://www.gcglaw.com/resources/entertainment/music-copyright.html
A few years back, Mike Boo wrote a good post on licensing, particularly the synchronization license as it affects drum corps videos, which led to a good thread:
http://www.drumcorpsplanet.com/forums/index.php/topic/152151-the-challenges-of-music-licensing-and/