By default PowerPoint saves your presentation as a .ppt or .pptx file which is the standard file type to save PowerPoint files in with .ppt being the older type. You can start your slide show for your presentation with a file saved as this type but it's not the best choice if you want to use your file as a slideshow exclusively.
PowerPoint has an option to save your presentation as a slide show if you want to be able to show your presentation without actually having to open it in PowerPoint and then going to the slideshow option within the program. This comes in handy if you are e-mailing your presentation and just want the recipient to be able to double click the file and watch the show.
To do this all you need to do is have your presentation open and go to File and Save As. In the Save as type drop down box you can pick PowerPoint Show (*.pps) . This will save the file with the .pps or .ppsx extension which designates it as a PowerPoint slide show. It will not overwrite your original .ppt file. You can also open ..pps or .ppsx files within PowerPoint to make changes.
The end user can simply double click the .pps or .ppsx file and start the slide show assuming they have PowerPoint installed. If you don't have PowerPoint you can download and install the free PowerPoint Viewer.