The base Playstation Network is free, but you have to provide basic info to sign up for it. PSN provides online matchmaking, friends lists, messaging, and voice chat in games for free, along with Playstation Home (a weird second life like environment). As to what info was mandatory, I can't remember now that it's been close to 5 years since I signed up. 77 million accounts existed here, for PSN or Qriocity access.

On top of that is the Playstation Store. It's a digital storefront that people can use to download free demos and videos, along with paid games, extra content for games, and TV/movie shows. It also handles subscription services, such as DC Universe Online, Qore and PSN Plus. 12.3 million accounts had credit card information saved.

To fund the store, Sony allows people to enter their credit card details to put money into a "Playstation Wallet". This wallet could also be funded via cards bought in retail stores, and I think possibly via PayPal and some other more direct debit services. This database containing the payment details was encrypted, but it's unknown if the hackers actually gained access to any information here. Sony still claims that they have not seen any reports from the credit card companies indicating fraud from this specific hack.