You may have noticed that all of your Outlook folders are located under one top-level folder, called Personal Folders. (This may not be the case if you are using Microsoft Exchange Server.) You can look at the photo below for an example of how personal folders look in Outlook.

 

Outlook stores all of your data (except rules, e-mail account settings, and interface customizations) in a personal folder file. This file always has a .pst extension, and can only be opened by Microsoft Outlook.