Apple’s App Store and iTunes are down in a bizarre service interruption that has caused practically all their uses to be unable to purchase anything using the services.
At the moment they appear to be fine as you can still surf though the listings on both stores and make the odd download (in some cases, not all) but you cannot actually purchase anything; and the issues have been happening since earlier today and have yet to be fixed. The Mac App store as well as their iBooks store are also still unavailable to make purchases as well, in one of the biggest service outs Apple has had this year.
Apple hasn’t said how far along it is in the recovery process, but you can check out which services are still functional here.