This is a great little summery of the different types of licenses offered.

Licensing is a very subjective issue, and is typically determined by the user of the music. The choice of licensing a cue can be based on such things as budget, expected use, and available options. How much a client will pay for various licensing is based many factors such as exposure, region, and whether the use is a royalty paying spot. For example, commercials and promos do not pay royalties therefore licensing is much higher. The different types of licenses are:

Blanket Licensing: available on a timed basis. One Month, Year, or perpetually. The longer the time the higher the rate.

Needle drop Licensing: based on each use of any cue. For example if a editor used the cue twice in a given spot, the composer and publisher are paid twice. That includes looping of any part of a cue. Each loop is a needle drop.

Production Blankets: licenses of multiple drops in a given production. One license covers as many drops as an editor, in a single timeframe broadcast or other use, wishes to use. For example: an episode of CSI.

Thanks to David Trotter - you can follow him here
http://www.linkedin.com/pub/david-trotter/a/61b/707