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It’s an interactive site you start off simply with some naming conventions and math equations inside an interactive "terminal" window. It saves your progress if you make an account, and you gain Foursquare style "badges" for completing lessons that you can share on your favorite social media site.
The depth of the lessons seems pretty good, and the site also have a feature to become a teacher once you log in, so if you want to contribute to the learning experience and add more lessons, you can start contributing right away.