This article had a little more info: http://krebsonsecurity.com/2013/12/sources-target-investigat...
The type of data stolen — also known as “track data” —
allows crooks to create counterfeit cards by encoding the
information onto any card with a magnetic stripe.
So it's not just the credit card numbers, but it's the full magstripe that was read and transmitted as-is to some central location where it was lifted. Debit cards and PINs, too, presumably.