Microsoft Office Open XML Format

Microsoft “Office 12” the new version office will introduce new default XML file formats for Microsoft Office Word word processing, Excel spreadsheet, and PowerPoint presentation graphics programs, and will change the way developers can approach solutions based on Office documents.The new Microsoft Office Open XML Formats introduce a number of benefits like interoperable , robust , efficient ,secured that will accrue not only to developers and the solutions they build, but also to individual users and organizations of all sizes.

Here is a nice place to find resources about open xml format document
http://openxmldeveloper.org/