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 .DOC extension represent documents generated by Microsoft Word or other word processing documents in binary file format. The extension was initially used for plain text documentation on several different operating systems. It can contain several different type of data such as images, formatted as well as plain text, graphs, charts, embedded objects, links, pages, page formatting, print settings and a lot others.
You can also convert PST into many other file formats. Please see the complete list below.