.pps files are created and opened using Microsoft's PowerPoint application. PowerPoint is a presentation program and is mostly used for displaying digital slides during business/educational presentations and is also widely used for displaying picture slideshows received by email.
Files using the PPS format can contain a combination of text, pictures and even movies. These files are all held within a single .pps file, so you only have one file to distribute or receive. The .pps file also holds information on how the presentation is displayed, such as the transition effects between slides.
To simply view a PowerPoint .PPS file you can download 'PowerPoint Viewer 2007', which is available free from Microsoft's website (link below). However, to create a PowerPoint slide-show you will need PowerPoint (which is part of Microsoft Office suite) which will also let you save your presentations in the .ppt (presentation) format, which is identical to the .PPS format except it will instruct PowerPoint to open the file in 'edit mode' (a .PPS extension indicates that the slideshow should be opened in 'Play mode').
PowerPoint is available for both the Windows and MAC OS operating systems and is now arguably the standard way of displaying presentation slides, making the slide projector virtually redundant.