A 'PPT' file is a Microsoft Powerpoint document. You can open them with PowerPoint for Mac, OpenOffice (and derivatives NeoOffice, LibreOffice, and Oracle Office), Apple Keynote, and Google Docs.
Open Office can be downloaded from OpenOffice.org and it is free.
Google Docs is web-based office suite. While it's a little awkward, it will permit you to import PPT presentations.
Most of them do a good job, but each will fall down with certain types of content. If the presentation contains ActiveX controls, they won't work on the Mac. Macros won't tend to work in anything but PowerPoint itself. Your mileage will vary on audio/video and embedded web content. There are also differences in transitions and animation between products that give peculiar results when going from one to the other.
If the presentation is a fairly typical one, however, all of the above will do a fair job of displaying it.