A PST file is a data storage file that contains personal information used by Microsoft Outlook and Exchange. It may also include e-mail folders, contacts, addresses, and other data.
PST files have a 2 GB limit and will cause Outlook to slow down when the file is close to the limit. Microsoft provides a program called the Exchange 2000 Tool: PST2GB that will reduce the file size of a PST file. However, the utility may truncate the file meaning some data will be lost, so you should make sure to back up the PST file first.
Unlike the full version of Outlook, Outlook Express does not use PST files. However, Outlook Express can import PST data using the program's Import feature. Mac users can use the PST Import Tool for Entourage to import PST data into Entourage or the PST to MBOX Converter to convert PST files to .MBOX files.