The public domain is basically the entirety of human knowledge and creation, freely accessible to anyone and everyone. But inventors and creators should be able to earn a living, so we have patents and copyrights. In the United States, these date back to the founding of our country.
Since that time, #copyright terms have been extended from 14 years to over a century. One of the most (if not the most) recent times was the Sonny Bono Act (a.k.a. Mickey Mouse Protection Act, Copyright Term Extension Act) in 1998. Furthermore, those extensions have often been retroactive, giving longer copyrights to creators than what they expected at the time they created their works.
Each time this happens, it robs the public of unfettered access ti what otherwise would have been in that public domain. And now it's about to happen again. The #TPP (Trans Pacific Partnership) is set to extend copyrights to creator's life plus 50 years!
It's being done in the name of harmonizing copyright laws, which I don't disagree can made international trade easier. I only wish that harmonization weren't exclusively a one-way, upward ratcheting.