Photopass is just the system that Disney uses when their photographers take pictures of you in the park. You will get the Photopass when you first have a Disney photog take a photo. It just looks like a credit card-- hard plasic, same size and shape.
Everytime after that when you have a Disney photog take your picutre you hand over your Photopass card. Then when you get home you go to the Photopass website (that information is on the card) and enter in the numbers and view your photos and you purchase.
The only thing you can do ahead of time to "plan" for this is to prepurchase the CD offer. You get a bit of a discount if you buy it ahead of time. But if you don't end up having very many pics taken then it would have been a waste of money. If you have lots and lots of pictures made in the parks then it would be to your advantage to have the CD prepurchased.
There are also options to view your photos while you are still in the parks and add frames and borders and other cute things, and even purchase there. You can also add frames and borders and things at home. I am not totally sure of all the different options.