Yes, they were CDS.  From what I recall, there was a discussion of how some sort of sharing system could be created and the end result was the burning of CDS.  I would have to go back and re-read to fully remember the "trees" part. 

One thing that was interesting to me was how people would add songs that they felt were vastly unknown and then, you'd receive a CD from someone across the globe, and the song you felt no one had ever heard was on someone else's burned CD.

Heck, now all one would have to do is create some sort of shared playlist on music platforms, and is not something that in older generations we had the grand excitement of buying a new LP, which has now been replaced with that of the reduced excitement over a "download."