Tech Talk

Offline folders (.ost) files are stored on your computer and are available even when the network is down. The default location for the .ost is your current Windows folder. Like personal folders (.pst) files, .ost files can be compacted to save space on your computer.

The difference between an .ost file and a set of .pst files is that the .ost file starts as a mirror image of your folders on the Exchange Server, and works in conjunction with the Exchange Serve during synchronization. A set of .pst files, on the other hand, is simply a storage location on your hard disk or a server other than the Exchange Server.

When you work offline, you can add, delete, and modify the contents of an .ost file exactly as you would with a folder on a server. For example, you can change and move items in your offline Inbox, send messages that are placed in your offline Outbox, and read your offline public folders. In the meantime, information on the server is still processing. You continue to receive new messages in your mailbox while other users can add, delete, and change items in public folders. However, you are not aware of these changes on the server until you connect to the network.

http://www.kelley.iu.edu/TS/ServicesSupp...
http://www.mail.nih.gov/user/faq/overlim...