As previously stated a .PST is an MS Office Outlook mail data file, not just used as a backup file, it's the place where Outlook stores the mail it downloads if you use POP mail, if you use IMAP then your emails are stored on the server.
You can import it into Outlook quite simply.
In Outlook 2010 click on the "File" tab then select "open" and then select "import" it will bring a box up the normally has "import from another program or file" highlighted click next then select "outlook data file (.pst)" then browse for your PST file. It will then import all the emails, contacts and folders from that PST file and put them into your current user settings. Don't forget to setup a mail account to import the emails into.