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 .XLT extension are template files created with Microsoft Excel which is a spreadsheet application which comes as part of Microsoft Office suite. Microsoft Office 97-2003 supported creating new XLT files as well as opening these. The latest version of Excel is still capable of opening these old format template files. Such a template file is used to quickly create new Excel files with default data and settings such as page formatting, font size, margins, charts, etc which can be further saved as new .XLS files.
You can also convert PST into many other file formats. Please see the complete list below.