There is no difference....maybe.
With that out of the way, I'll try to explain. First, we're talking about the full Outlook; not Outlook Express. Outlook can use several diffferent sources for it's 'Address Book'. The most common source of information for the 'Address Book' is Outlook's Contacts folder. But, depending on the configuration, Outlook could also use the Windows Address Book (comes with Windows), a Global Address book (Exchange Server), and a couple of other sources as it's 'Address Book'. So, to answer your question - it depends on the configuration.
Hope that helps.