Linux, or GNU/Linux, is the name for the software at the heart of operating systems like Ubuntu, RedHat, Slackware and Arch. Most Linux-based operating systems are available free of charge, and the creators hope that you'll use it, like it, and share it.
Linux-based operating systems power a majority of the modern Internet at a fraction of the cost of their proprietary competitors. Other UNIX-based operating systems, like Mac OS-X, are so similar to Linux that skills in one can be generally applied to the other.
"Hackett and Bankwell" is a fun way to get started learning Linux. In two quick episodes, you'll be up running in both the graphical and command line environments, and you'll be well-versed in the history of free software and its vocabulary.