The extension .PST refers to files of the type, Outlook Personal Information store files. The extension is the short form for Personal storage table file.

These are the data files that are created by applications like Microsoft Outlook and the Windows messaging client from Microsoft. The format is an open specifications format which is currently controlled and maintained by Microsoft. The file structure includes personal contacts, emails, addresses and many other details that the user wants to store. The maximum limit up to which the data can be stored in a PST file is 2 GB.