“Whereas PowerPoint uses a binary format to store the information for a presentation, Apple chose to create a publicly documented XML vocabulary to store Keynote presentations, APXL (short for Apple Presentation XML). While XML is used extensively in OS X (Cocoa applications use XML to store application preferences and cross-platform server-side applications such as Tomcat and Cocoon use XML for configuration files), until now, it has never been used by Apple as the sole method to store documents.”
[full article at xml.com]