Now, the captchas provided by the site aren't very "hard" to solve (in fact, they're downright bad):

But there are many interesting parts here:

1. The HTML 5 Canvas getImageData API is used to get at the pixel data from the Captcha image. Canvas gives you the ability to embed an image into a canvas (from which you can later extract the pixel data back out again).
2. The script includes an implementation of a neural network, written in pure JavaScript.
3. The pixel data, extracted from the image using Canvas, is fed into the neural network in an attempt to divine the exact characters being used - in a sort of crude form of Optical Character Recognition (OCR).

Oh Javascript devs, what will you think of next.