An offline folder file is the location for offline folders (OST files) in Microsoft Outlook. Offline folders make it possible for the user to work offline and then synchronize updates with the Exchange server when they next connect to the network. Changes made offline are copied to the Exchange server during synchronization, after which the user may revert to working offline. The ability to work offline is useful in environments with limited or unreliable connectivity.
Offline folders differ from PST files (.pst files), which are personal folder files stored either on a local disk or a server other than the Exchange server.