A Personal Folders file (.pst) is an Outlook data file that stores your messages and other items on your computer. This is the most common file in which information in Outlook is saved by home users or in small organizations. The most common types of accounts are referred to by their Internet protocol names -- POP3 and IMAP. Another type of account is an HTTP or web-based account that works similar to IMAP email accounts. All three account types use a .pst file.