The applet implements ``diffusion limited aggregation'' (DLA). The process goes like this: start with a single `molecule' sitting all by itself (center of screen---that little dot). Then other molecules randomly fly around. If they hit that lone molecule, they stick. Just out of this simple rule, over a long period of time, you get interesting looking patterns.
In this special ``tetris'' vesion, the molecules are just tetris blocks flying around.
Pretty amazing what randomness can do. Here are some screenshots in case you can't wait: