Photopass is amazing! When you enter a park and have your first picture taken either with a character, in front of the castle, Sorcerors Hat, EPCOT ball etc the photopass photographer will hand you a credit card type card. You will then hand this card to the photopass photographer every time you have a picture taken. They scan the card and the pictures are uploaded to an account (the code to access this account is on the back of the card - so write the number down somewhere safe so that even if you lose the card you can still access the pictures).
When you come home (or even while you are there) you can log onto the photopass website and see all your pictures. You can then either buy individual ones or have them all uploaded to a CD that they will post you (that way you can print them yourself say at Boots very cheaply).
I just got back this month and I had a picture done with practically every photographer we saw

We ended up with 300 pictures on our card. The CD costs the same how every many pictures you have on it so best to get it out everywhere!!
The first park we visited this year was blizzard beach so they gave me a tiny little card that was on an elastic band that went round my wrist so you didn't lose it in the water. I thought it was great so popped it on my wrist everyday - that way it was always to hand when we had a photo taken.
On the photopass website you can also make photobooks (you can also upload your own photos to use), gifts etc etc.
I think you can possibly order a CD before you go for about $99 which is cheaper than ordering when you return.
It really is a great system. WE had some lovely photos on ours. I will take a photo of our photobook which I just got last week and upload them tonight for you.