Personal Storage Table (PST)
A PST file, or personal storage table (.pst) file, is a Microsoft Outlook Data File that stores a user's Outlook data for POP3, IMAP and web-based mail accounts, including all mail folders and the items within the folders, such as emails, email attachments, to do items and appointments, contacts and more.
Personal storage table files are used by all versions of Microsoft Outlook, from Outlook 2000 up to Outlook 2010. PST files in early versions of Outlook (Outlook 2000 and Outlook 2002) were stored in ANSI format and have a maximum size of 2GB, while later versions save the PST file in Unicode format and have a maximum size of 20GB (Outlook 2003 and Outlook 2007) or 50GB (Outlook 2010).
For Windows Vista and later OSes, Outlook PST files are stored by default in C:\Users\%username%\AppData\Local\Microsoft\Outlook\ for Outlook 2007 and in C:\Users\%username%\Documents\Outlook Files for Outlook 2010.
PST files can be compacted using the "Compact Now" option in Outlook's Data File Management area, and corrupted PST files can be repaired using either Microsoft’s ScanPST application or a third-party application.
A similar Outlook data file known as an OST file, or offline storage table (.ost) file, functions as a cache for Microsoft Outlook's Cached Exchange Mode feature. With OST files, Microsoft Exchange Server and Microsoft Hotmail (via the Outlook Hotmail Connector) store archived versions of a user's Outlook or Hotmail data on a local computer instead of on the mail server in order to provide availability to these items when access to the server is unavailable.
Unlike PST files, OST files are tied to the mail account configuration and cannot be opened or backed up individually.