The purpose of the game is to complete tasks, such as drawing a triangle around a circle, using only Eucledan tools. All you have is a straight edge and a compass. This game teaches you how to construct more complex drawings based on these simple tools.
The first tasks are simple, and it gets progressively harder in later levels. The first levels are free, and then it's a small amount of money (1 USD?) to unlock the rest. I paid immediately as I completed the free levels, as did all that I recommended this game to.