The main page starts you off with a vocabulary game, but you can choose from about ten subjects and switch subjects at any time. The game monitors how you do compared to a database of results from thousands of other players and adjusts the difficulty accordingly. This keeps it interesting and makes it a powerful learning tool since it adjusts to your knowledge level.
If you miss a question you will see the correct answer and be asked the same question a few turns later to help reinforce it. The game is much more addicting that you would surmise from and initial perusal. I can't think of a better way to waste some time.