So, there are a couple of different options, Just a regular Photopass is totally non-committal. You get picts taken around the park, they scan your card (which you get the first time you have a pict taken), you view the photos in the photo store or online and can order individual photos or a CD of the photos. You can add cute Disney borders to the photos on the CD, if you like.
Photopass+ is a little different. You prepay for it either by preordering (at least 14 days before your trip - they mail you a voucher to redeem at the park) for $69.99 via the link another poster listed above, or you can purchase at
Disneyland for $99.99. It includes a code to order the CD of your photos for free, a CD of stock photos, and a voucher to redeem for a lanyard in the park. The lanyard allows them to add the digital files of the rides photos to you account for free and gives you the digital files/images from the character meals free also (it is valid for as many of those as you like for 14 days).