http://www.linux.com/whatislinux/114354 - that should give you a good starting point and explains it better than I could.
A very brief eplaination: a free, from the ground up, UNIX work alike invented by Linus Torvalds which subscribes to "open source" philosophy. Check wikipedia for linux, unix, and linus torvalds for more info.
Thanks guys! Now off to see what this is all about...