PowerPoint 2010 and 2013 presentations use the .pptx file extension. PowerPoint 2010/2013 are based on Office Open XML formats (but you do not need to know how to use XML to work in PowerPoint 2010/2013).
The reasoning behind this change is because XML helps keep presentations safer by separating out files containing macros or scripts, keeps presentation sizes smaller, and helps to prevent data corruption and loss by making presentations less susceptible to damage.
You can open presentations that were created in previous versions of PowerPoint. When you open such a presentation, it will open in Compatibility Mode.
When you save a file that was created originally in a preview version of PowerPoint, the Save As dialog box will open automatically, to save the file with a .pptx file extension.
Also, when you save a PowerPoint 2013 file as a previous version, Compatibility Checker will identify any new features that will not work in that version.