A typical Linux system is largely comprised of unrestricted, open-source software. But some of the most popular software around is either closed-source or open-source, but restricted by patents. Depending on your Linux distribution, this forbidden software may be easy to install--or require jumping through some hoops.
This stuff is roped off for several reasons. Many Linux distributions don't want to distribute closed-source software, so the AMD and Nvidia graphics drivers
you'll need to play 3D games with maximum performance aren't easily available, and neither is software like the Flash plug-in. Other Linux distributions don't want to violate software patents in their home countries, so they don't distribute open-source multimedia codecs that allow you to listen to MP3s or watch MP4 videos.