It was mainly the forums... but the database that whoever it was had access to did have more information... even though it was encrypted, it may eventually be compromised *if* they have copies of it...
In regards to account details being hacked - they're saying that if you use the same username/password combo on Steam as the forums, then go change it.
In regards to losing your account though, as this happened with one of my first ones, the only thing you need for proof is a list of games purchased with exact prices and dates. So always print off Steam receipts just incase you ever do lose your login details.
First of all, the passwords are encrypted, it would take years to crack even simple passwords. And yeah, thank god for the Steam guard. If you are really afraid of loosing your account, just change your password.
I always make sure to leave "save my information for faster payment" unchecked, I hope it really isn't saved. I'd love to have this option in the iTunes store too.
Gabe has just stated that all CC details are AES256 encrypted so, as long as there hasn't been a grotesque lapse in security and the keys leaked, they should be safe.