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 PST files can be stored anywhere in your computer and it makes it that much easier for you to keep a backup copy of your PST files or move it from one computer to another.
