There is a good article regarding Offline Address Book (OAB) on Microsoft's support site.
From article's summary:
Microsoft Office Outlook 2007 and Microsoft Office Outlook 2003 have many new features and enhancements that are related to the offline address book. By default, Outlook uses a cached mode configuration. Because cached mode generates an offline address book, understanding how to configure the offline address book so that it works efficiently in your Microsoft Exchange organization is important.
Article can be found here.