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.
A file with DOTM extension represents template file created with Microsoft Word 2007 or higher. It is similar to the popular DOCX file format other than it retains the user defined settings for reuse in case of creating new documents. Such documents are more often used in offices where a standard template file is generated with settings like page information, margins, default layout and macros, and is used to create new documents from it when required. DOTM files, however, save macros, that are a series of commands in the form of recorded actions for automatic completion of a task. This helps save time in carrying out actions that are repeated in completion of a task.
You can also convert PST into many other file formats. Please see the complete list below.