Basically, there are two categories of Outlook Data files used by Microsoft Outlook. An Outlook default Data File (.pst) is used for most profiles. When you use Microsoft Exchange account, then your content generally delivered to mail server. Microsoft Outlook also allows you to work with your mails even when you are not connecting to the mail server, another type of data file that is called an (.ost) offline Outlook Data File is kept on your system.
Outlook Data Files (.pst) — A Personal storage type (.pst) is an Outlook data file that keeps your mails and other things on your system. This is the most common file format in which data in Outlook are kept by home users or in a small business. Home users are commonly using an Internet facility provider (ISP) to connect the Internet. The ISP also provides more than one email account. The very most common types of accounts are referred to by Internet protocol names � POP3 and IMAP. Second type of account is an HTTP, HTTPS, or web-based account that works like to IMAP email accounts. All three account types use a .pst file.
PST are used for POP3, IMAP & web-based account profiles. When you need to create archives or backup your Outlook data folders and objects on your system, such as Exchange accounts, you must generate and use further .PST files.
Outlook Data Files (.ost) - Usually, when you use an Exchange Server account, your messages, calendar, contacts and other data are stored on the server. You can configure Outlook account to keep a copy of your data on your system in an Outlook data file which is named as (OST) offline Outlook Data File. This will allow you to use work offline when a connection to the MS Exchange server computer is not possible or required. The .ost file is coordinated with the Exchange server computer when a connection is accessible.
This type of file is also used for accounts/profile that you set up along with the Outlook Connector for Outlook.com. Offline Outlook Data Files (.ost) are continually copies of data that are kept on a mail server and don�t have to be backed up like Default Outlook Data Files (.pst).