I read this Wikipedia article about it, but what I want to know is how to play it in the first place.
How do I draw walls? How do I stop them? How do I control them?
Here, have a commented screenshot of the original game.
If you really have too much time on your hands, this how-to explains how to exploit the game "physics" to complete levels of any difficulty.
Your goal is to fill 75% of the board.
If you create a closed line (i.e. a square) with no balls in it then it gets filled. You can build vertical and horizontal walls (right click to choose wall direction), the wall will be built starting at your mouse position and propagates from that point in two direction, suppose you're building an horizontal wall then it will propagate on the right and on the left.
You have to be careful because wall-building gets interrupted by balls' collision, and that's the game.
In some versions if a ball hits a head of a propagating wall the wall stops to grow and the ball bounce, if a ball hits a side of a propagating wall it destroy that wall and you lose a life.