Files with .PST extension represent Outlook Personal Storage Files (also called Personal Storage Table) that store variety of user information. User information is stored in folders of different types that include emails, calendar items, notes, contacts, and several other file formats. PST files are used for archiving emailing data offline that can be later loaded and viewed in various applications.
Files with XLS extension represent Excel Binary File Format. Such files can be created by Microsoft Excel as well as other similar spreadsheet programs such as OpenOffice Calc or Apple Numbers. File saved by Excel are known as Workbook where each workbook can have one or more worksheets. Data is stored and displayed to users in table format in worksheet and can span numeric values, text data, formulas, external data connections, images and charts.
You can also convert PST into many other file formats. Please see the complete list below.