Outlook stores your information in various locations, depending on how you or your network administrator configured Outlook. The location where your personal information is stored is called the primary store.
In most cases, the primary store is a Personal Folders file on your hard drive. Personal Folders files are often called .pst files because .pst is the file name extension.

Most of the time, you can leave Personal Folders files in the default location selected by Outlook. But you can move Personal Folders files to new locations, as well as create additional files in the Outlook Data Files dialog box.

Why create additional files? Well, if you upgraded from an earlier version of Outlook, your system probably uses the old style of Personal Folders file. In addition, while Personal Folders files can hold gigabytes of data, they can be difficult to manage after they reach a few hundred megabytes. Some people create new Personal Folders files every few months to simplify the task of backing up and maintaining the files.

To view information about your Outlook data files or create new files, follow these steps:

1. In the Outlook main window, click File | Data File Management to open the Outlook Data Files dialog box.
2. Select the Data File you want to work with and then click one of the buttons along the right side of the dialog box to make any changes.
3. Use the Settings button to view General Information about the Data File, including its location and format.
4. Click Compact Now if you want to reduce the size of the file by compacting (or compressing) it.