.pst is a filename extension in Microsoft Windows operating system. If one uses Microsoft Exchange Server, the messages, calendar, and other items are delivered to and stored on the server. If one does not use Exchange Server, the messages, calendar, and other items are delivered to and stored locally in a personal folders (.pst) file.

When one creates messages, appointments, tasks, and journal entries, Microsoft Outlook saves the information in a personal folders (.pst) file on the computer.

The data file is called a personal folders (.pst) file because it is saved locally with a .pst file extension. The personal folders (.pst) file is the same as in previous versions of Outlook, and is compatible with Microsoft Outlook 2000, Microsoft Outlook 98, and Microsoft Outlook 97.