Linux is an Open Source operating system available under GNU Public License. I think it was released under GPL version 1, which would be listed under "Old versions". The GNU license basically means that it is free for all to use, open source means that is if free for all to modify. This is why there are so many flavors (distributions) of Linux -- Red Hat, Fedora, SUSE, Ubuntu, and many others. While the core of Linux (the kernel) is free, some companies like Red Hat, and SUSE add proprietary components and sell it as a value added Linux.

You can download free versions of most Linux distributions. Fedora is one of the more popular for servers and Ubuntu is one of the more popular for desktop environments. These are rather large downloads and not everyone can download such large files. For these people there are companies who burn and sell these CDs, or DVDs for cheap (like $5 or $10). My favorite supplier is OSDisk.com, located in Colorado.

Although Linux started off as an operating system that was only for very technical users, it has matured into a very user friendly and operating system with a graphic interface that everyone is accustomed to seeing.