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 PPT extension represents PowerPoint file that consists of a collection of slides for displaying as SlideShow. It specifies the Binary File Format used by Microsoft PowerPoint 97-2003. A PPT file can contain several different types of information such as text, bulleted points, images, multimedia and other embedded OLE objects. Microsoft came up with newer file format for PowerPoint, known as PPTX, from 2007 onwards that is based on Office OpenXML and is different from this binary file format.
You can also convert PST into many other file formats. Please see the complete list below.