Just to expand on what wayneg said a little - it seems to be photopass in general you want to know about?
When you enter your first park you'll see the photopass photographers straight away - there are usual several right near the entrances.
Approach one of them and they'll ask if you already have a card - which you wont have. They'll give you a credit card sized photopass card which has a number and barcode on. They will then scan the barcode which associates the pictures they take to your card. Once you've had your pictures take you can then wonder off to find other photopass photographers either in that park or another disney park. From then on just give them the card to scan and all the pictures will be associated with that card.
You can then either take that card into the appropriate shop within each park where you can view the photos and get prints/a cd etc or you can wait until you get home and do the same through the website.
There is also the option of having a photopass photo session at most of the deluxe disney resorts (AKL, Poly and GF for example) where by you book a session and the photographer will take you to several key locations at the resort and take pics of your family. However there is an additional cost (some say a slightly too high price!) to this, and the rules of what you get and whether you can take the photopass card away afterwards keep changing so I'm not sure what the latest is on this!