This is just a collection of videos that I found about FOSS. For now you can watch: The Codebreakers (2006), a BBC documentary and Revolution OS (2002) a nice documentary featuring many people from the Free Software movement and Open Source.
The short answer is: Linux is the kernel of a free operating system that suits as a replacement for Windows or Mac OS. Both Windows and Mac OS are proprietary software, they are available for sale and the user cannot modify how these operating systems work. The Linux kernel and the GNU applications that run on this alternative operating system are (usually) free of charge and free to be modified by the user or any community of developers. It is built by the users, for the users.