If you've been using ebooks for any length of time then you've probably encountered DRM.
DRM is why you can't convert an Epub ebook so you can read it on the Kindle. DRM is why you don't actually own the ebooks you bought. DRM can restricts your access to movies, music, literature and software, indeed all forms of digital data. It is simply a prison in which users can be put to deprive them of the rights that the law would otherwise allow them.
Naturally, a lot of us aren't happy about that.
The recent HarperCollins decision to restrict library ebooks has given new strength to the anti-DRM movement, and it inspired Nina Paley to draw these icons. They're CC licensed, so feel free to use them wherever. You can find several other versions at readersbillofrights.info.