PhotoPass starts out as free. There are photographers in the parks who will take your photo. There is no charge for this. The photographer will hand you a card with a code number and bar code on the back of it. Every time you want your photo taken, simply hand the card to the photographer and all your photos will be added to one account. (You can have multiple cards going at the same time.) You can do this as much as you want with no cost or obligation.
Once you get home (within 30 days), go to disneyphotopass.com and type in the numbers that are on the back of your PhotoPass card(s). You can see your pictures here and decide whether you like them. Everything up until now is still free. If you like the pictures, you can decide to buy them. (Now comes the not-free part.) Prices can change, but a single 5x7" photo costs about $12. If you have more than about ten photos that you like, you can purchase the Disney PhotoPass CD. Again, prices can change, but the CD costs about $125. With the CD you are given the digital files of all of your photos (several hundred at least) for the one charge. With the CD you are also given the rights to the pictures so you can print them out as you want or make them into holiday cards, etc. There are often pre-purchase discounts of about $25 on the CD if you buy it before your trip rather than after. More on this
HERE.
To recap: There is no charge or obligation to have your photo taken or to look at the photos online. If you want to keep the photos, you do have to buy them. You can buy either individual prints for about $12 each or you can have hundreds of digital files on CD for about $100.
The best PhotoPass resource I know of is at the independent website stitchkingdom.com. Look for his wonderful "Definitive Guide to PhotoPass" for more info.