Ubuntu is a Linux based operating system by the community and a company called Canonical. The system is easier to use than most Linux distrobutions in that it has a very intuitive interface and some good support if things do go wrong. They rarely do. Ubuntu is a stable, open, and cutting edge Linux operating system that most people can use if they give it a chance.
Check out a great review for Ubuntu 12.04 that just came out by ZDNet.com:
Here is a guide to download and installation: