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.
Introduced and documented by Microsoft, the Rich Text Format (RTF) represents a method of encoding formatted text and graphics for use within applications. The format facilitates cross-platform document exchange with other Microsoft Products, thus serving the purpose of interoperability. This capability makes it a standard of data transfer between word processing software and, hence, contents can be transferred from one operating system to another without losing document formatting.
You can also convert PST into many other file formats. Please see the complete list below.