users use a Microsoft Exchange Server account, their all
emails messages, calendars, contacts and other items are distributed to and kept on the server. Mostly, users configure Outlook to keep a local copy of their items on their computer in an Outlook data file that known as OST (offline storage table). This file allows working in offline when a connection to the exchange computer may not be possible .
PST (Personal Storage Table) is an Outlook data file that keeps users messages and other items on their computer. Small organization and home users used to save the information in outlook PST file.