Photopass is a system WDW has set up where you get a card from a photographer and you can use that card to associate pictures they take of you for the rest of your trip. It's pretty convenient not to have to carry a camera around and only a little card - until it's time to pay for the photographs. Then the air is let out of the balloon.
There's no way to download these pics from the website they're kept on. You have to purchase printed pics at a very high cost and have them mailed to you, or pay $99 for a CD with all of your pics on them, and you still can't download them. I'm sorry, but $99 is absolutely ridiculous to pay for a 25 cent CD and a bunch of digital photos that I could have just as easily taken for free with my own digital camera.
We used Photopass pretty heavily on our last trip, and regret it heavily. Not only were the photos not any better than we would have taken with our own digital camera, but the outrageous costs were not in our ballpark of what we consider fair pricing for the amount of work that went in for this, which is very little, when you think about it.
If you already have a digital camera, I strongly suggest you bring it with you.