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.
DOCM files are Microsoft Word 2007 or higher generated documents with the ability to run macros. It is similar to DOCX file format but the ability to run macros makes it different from DOCX. Like DOCX, DOCM files can be store text, images, tables, shapes, charts and other contents. The capability to run macros make it easy to save time by executing the series of commands in the form of recorded actions for automatic completion of a task. DOCM files can be opened and edited in Microsoft Word 2007 and above.
You can also convert PST into many other file formats. Please see the complete list below.