The photopass is a card you can use the entire trip and they "scan" it when you have a Disney photographer take your picture at the various spots (in front of the castle, Mickey's house, etc.)
You can view your pictures online when you get home or at the photo stations in each park. You use the same pass every day and in both parks.
The voucher is good for a free 5 x 7. As I understand it, some people have actually used there voucher for any picture on their photopass, not just the one from the meet and greet. We did not try this, we wanted that picture! They give you the voucher when they see your card and they take several poses for you which they "put" on your photo pass card for you.
They had a special going in May where we paid $70 for all the pictures on the card (up to 300 I think) and you get the release to print them anywhere you want. We also got 3 5x7 prints that they did for us in the park.
I REALLY recommend photopass. We were very happy with it and really wished we had used it a lot more while we were there.