The applet implements ``diffusion limited aggregation'' (DLA). It's somewhat similar to the other ``snow flake'' applets somewhere around this site---except this one implements a `random walk' instead of just shooting things from random directions. The results are somewhat similar though.
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 (randomly changing direction all the time). 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.
Pretty amazing what randomness can do. Here a screenshot in case you can't wait (it does take a -while- for this thing to produce anything interesting: