Content Scrambling System
Content Scrambling System (CSS) is an algorithm used to encrypt commercial DVDs in an (ultimately futile) attempt to prevent them from being copied. CSS also enforces restrictions such as region control.
The open source DeCSS program can decrypt CSS-protected DVDs under Linux, but is controversial because it violates the DMCA in America. In most Linux distributions CSS decryption is provided through the libdvdcss library, which allows media players such as xine and MPlayer to play encrypted DVDs with no further configuration required.
Due to legal issues, Linux distributions tend to ship without any CSS decryption software installed, so it is necessary to download and install the libdvdcss library manually.