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.
PPS, PowerPoint Slide Show, files are created using Microsoft PowerPoint for Slide Show purpose. PPS file reading and creation is supported by Microsoft PowerPoint 97-2003. The more latest version of this file format is PPSX which is based on Office OpenXML standards. PPS files can still be read by latest versions of Microsoft PowerPoint, but newly created files can only be saved in PPSX file format. When a PPS file is shared with another user and opened, it starts as Powerpoint show unlike PPT file which opens in editable mode.
You can also convert PST into many other file formats. Please see the complete list below.