Outlook Data File (.pst): (.pst ) Data file basically stores all the emails and other Outlook items on your computer, this folder is very useful if you are wish to take a backup of all your outlook data.
The easiest way to find out (.pst and .ost) folder location is through Outlook itself:
1. In Outlook 2010, click the File tab.
2. Click Account Settings, and then click Account Settings (Add or Remove accounts or change existing connection settings).
3. Click on Data Files tab, select any id, and then click Open File Location.
For Outlook 2010 Users
Recently, many Outlook 2007 users migrated to Outlook 2010. Outlook Data Files (.pst) created by using Outlook 2010 are saved on your computer in the Documents\Outlook Files folder (You will find your email ids)
Windows 7 Outlook Office 2010 pst file location is different than Windows XP
If you upgraded to Outlook 2010 on a same computer that already had Outlook 2007 or earlier version then the file location of .pst different.
In Windows 7 and Windows Vista file location is : drive letter:\Users\user\AppData\Local\Microsoft\Outlook
Just copy and paste this in the Search field on the start menu to open windows explorer to the default location of the pst files: