The first time you get you photo taken by a PhotoPass photographer, they'll give you a card. Each time you get your photo taken after that, hand the photographer your card and they'll scan it to add your new photos.
This is all free - no hidden costs at all!
Then you can either view your photos in the parks, or online. It doesn't cost anything to "claim" your photos - you can view FOC as much as you like for approxiamtely a month. You will only pay if you order any photos/photo CD/PhotoMovie/PhotoBook.
You can add multiple cards to your account (so don't worry if you forget your card one day - you can just get another).
TIP - keep a note of your card number(s) - if you lose/misplace your card(s) you will still be able to view your photos.
Hope that helps.