GNU/Linux is a free software operating system that runs on just about any modern computer.
GNU/Linux (often called just "Linux") is made up of many small programs (mostly created by the GNU Project) and the Linux Kernel.
Here's some places you can get GNU/Linux, support and more information:
http://www.debian.org/ (Good for non-programmers, and for getting started)
http://www.gentoo.org/ (For experienced GNU/Linux users who want more control over what options things are compiled with etc.)