Difference between MS Outlook OST & PST files  OST stands for
Offline Storage Tables. It is an offline folder created by MS
Outlook and MS Exchange to hold offline copies of whatever
exists in the Exchange server. To make it more simple to
understand, you can just call OST files as as local copy of your
Exchange mailbox. It is created when your emails are delivered
to your Outlook mailbox and will contain all your emails,
contacts, calender entries and tasks. Outlook has to be in
Cached mode for Offline folders to work. OST files allow you to
work even when you are not connected to your exchange server
(i.e. when you are offline) and then, when you get online it
will synchronize itself with your exchange server mailbox.  On
the other hand, PST stands for Personal Storage Tables. These
are also known as personal folders or personal files. It is not
created by Outlook or the Exchange server. Instead, you (or any
user, for that matter) can create it to store all your emails,
contacts, calendar entries etc. These