ECMA International Approves Office Open XML Standard

Check this for the press release -
http://www.ecma-international.org/news/PressReleases/PR_TC45_Dec2006.htm

Microsoft Office Open XML (OOXML) is a specification for a file format for the storage of electronic documents such as memos, presentations, and spreadsheets. The specification was developed by Microsoft and is used in Microsoft Office 2007.
Microsoft's Office Open XML format uses a ZIP container for packaging XML and other data files. The resulting files are smaller than the binary files created by the previous Office formats. Microsoft maintains that its primary goal has to be backward compatibility with existing documents and full support of its extensive feature set. The Microsoft Office Open XML format is Microsoft's direct answer to the OpenDocument format (ISO/IEC DIS 26300) which was created by the OASIS foundation and uses similar technologies (XML contained in ZIP). A comparison can be found in Comparison of OpenDocument and Microsoft XML formats.

Source - http://en.wikipedia.org/wiki/Microsoft_Office_Open_XML