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.
Text files created with Markdown language dialects is saved with .MD or .MARKDOWN file extension. MD files are saved in plain text format that uses Markdown language which also includes inline text symbols, defining how a text can be formatted such as indentations, table formatting, fonts, and headers. MD files can be converted to HTML with a program called Markdown. Markdown language is released by John Gruber.
You can also convert PST into many other file formats. Please see the complete list below.