By default, documents created in Microsoft “Office 12” will be based on an XML file format definition. The Open XML Format is based on XML and ZIP technologies, thereby making it universally accessible. With industry standard XML at the core of the Open XML Format, exchanging data between Microsoft Office applications and enterprise business systems is greatly simplified. The Open XML Format has been designed to be more robust than the binary formats, and, therefore, will reduce the risk of lost information due to damaged or corrupted files. New file extensions will be *.docx, *.pptx, *.xlsx etc. More details please refer Biran Jones’s (Program Manager of Word) blog.