The new 2007 Microsoft Office system file format is called Open XML Formats and applies Word, Excel, and PowerPoint.
New file formats for the 2007 Office release
The default 2007 Microsoft Office system file format is new and is not compatible with earlier versions of Microsoft Office. For Word 2007, Excel 2007 and PowerPoint 2007, the new default extensions are .docx, .xlsx and .pptx. These new formats help:
Increase file security
Reduce the chance of file corruption
Reduce file size
Improve data sharing across a range of storage and retrieval systems
The 2007 Microsoft Office system can open files created in earlier versions of Microsoft Office. The files are opened in Compatibility mode, and you know that you are in Compatibility mode when you see (Compatibility mode) after the file name in the title bar.
For an overview of the Open XML Formats, see Introduction to new file name extensions and Open XML Formats.