I can't really tell you what the Extreme Music Library is about.
I think they offer music to professionals, like movie makers or companies who make advertisments.
Check out their website and be amazed:
Some of these compositions (or maybe even all of them) are also available on promotional CDs.
And now the fun part kicks in: Dweezil Zappa contributed a number of pieces to the Extreme Music Library, and these have been released on a number of CDs.
Data, or at least some of it, has been added to the United Mutations Archives.