PowerPoint XML and RibbonX
Why XML in PowerPoint?
Since Office 2007, PowerPoint and the other Office programs use XML as their internal language. XML is quite similar to the HTML you may already be familiar with, except that it's much stricter about structure, capitalization and such, and it's extensible.
This isn't the place for an XML tutorial; there are plenty of those on the web. Instead, we'll list some resources that will be useful if you want to modify the XML in PowerPoint files (or any of the other XML-based Office files: Word, Excel and so on).
How to edit PowerPoint's XML
As mentioned, PowerPoint files are now in XML format. Or more accurately, they're a host of XML files inside a standard ZIP archive file. Editing one of the component XML files is a matter of extracting it from the ZIP file that has a PPTX/PPSX/PPTM/PPSM/etc. extension, editing it, then putting it back into the ZIP/PPTX.
Because I'm already tired of writing "ZIP/PPTX/PPSX/PPTM/PPSM/etc." let's assume from now on that when you see PPTX, you'll think "He really means all of the new XML file types. Humor him."
Thank you.
Now, while you can certainly work with PowerPoint XML using just the tools that come with Windows (Notepad, File Manager) or MacOS, you really do NOT want to.
Instead, Windows users should install 7Zip (free!) and Notepad++ (also free) and the XML Tools plug-in for Notepad++ (using the plug-ins manager from within Notepad++ once it's installed).
7Zip can open PPTX files directly without your having to rename them with a ZIP extension and can be configured to open the XML files within in any editor you like. In this case, Notepad++.
To edit the XML in a PPTX file, right-click the file in Windows Explorer, choose Open Archive (7-Zip has added this to the pop-up menu). Navigate through the files and folders within the PPTX (actually a ZIP file) to select the one you want to edit.
Next, set 7-Zip up to use Notepad++ as its editor. You'll only need to do this once:
Choose Tools | Options | Editor then enter or browse to wherever Notepad++.EXE is installed. In my dialog box, it says "C:\Program Files (x86)\Notepad++\notepad++.exe" -multiInst. The -multiInst bit is important; without it, files you edit won't be correctly saved back into the original PPTX file.
Once your chosen XML file is open in Notepad++, press Shift+Control+Alt+B to invoke "prettyprint" mode. Or to humans, individual editable lines of XML code rather than one or two lines that stretch from your monitor to China.
OK, now what?
Now that you know HOW to edit PowerPoint XML, what do you do with this new knowledge?
Here are some resources to get you started:
John Korchok has a wealth of PowerPoint and Word XML information on his OOXML Hacking site including good info on editing PPT XML on a Mac, which I neglect here. And if you're really into the whole XML thing, you'll want his book, OOXML Hacking.
PowerPoint MVP Echo Swinford has tutorials and other good info on her site. And if you develop templates for yourself or others to use, you're working way too hard and not nearly smart enough if you don't have Echo and Julie Terberg's Building Powerpoint Templates book.
Ron DeBruin has a wealth of Windows and Mac Excel/PowerPoint development information, including useful XML tips.
XML to define Ribbon features
If you're writing VBA macros/add-ins for PowerPoint, you may have no interest in editing the XML inside PPTX files, but sooner or later you'll want to add buttons/tabs and other "furniture" to the PowerPoint ribbon in order to more easily use your macros.
To do that, you'll need to add a special variant of XML called RibbonX to your PowerPoint files that contain VBA macros. You can do this with manually edited XML but once again, you really REALLY don't want to do this. There are special RibbonX editors that take care of the fussy bits that will make your life a misery otherwise.
Fernandreu's Office RibbonX Editor does the job superbly and is free.
The absolute bible & ultimate source of RibbonX how-to is RibbonX: Customizing the Office 2007 Ribbon by Robert Martin, Ken Puls, Teresa Hennig. It was written to cover Office 2007, but the information's still valid in current versions (though there are a few new Ribbon features added in later versions).
Ribbon Recipes by Craig Hatmaker focuses on Excel, but nearly anything that works in Excel's RibbonX will be valid in PowerPoint as well.