When the Disney photographers take your picture (like on mainstreet with the castle in the backround) they give you a plastic card. With this card you can either look at and purchase your pictures at the park if you go to the photo center, or you can look at them and purchase what you want on the photopass website once you return home.
You keep this card through your trip and hand it to the next photographer, that way all of your pictures will be listed under the same card's number.
We purchased some pictures this way in March. Some posters have commented that their photopass pictures weren't worth purchasing. So, if it is a photo that you really want, you may want to take your own as a backup.
I hope I've explained it clearly.