Office Open XML (also informally known as OOXML or Microsoft Open XML (MOX)) is a zipped, XML-based file format developed by Microsoft for representing spreadsheets, charts, presentations and word processing documents. The format was initially standardized by Ecma (as ECMA-376), and by the ISO and IEC (as ISO/IEC 29500) in later versions. 

Starting with Microsoft Office 2007, the Office Open XML file formats have become the default target file format of Microsoft Office. Microsoft Office 2010 provides read support for ECMA-376, read/write support for ISO/IEC 29500 Transitional, and read support for ISO/IEC 29500 Strict. Microsoft Office 2013 and Microsoft Office 2016 additionally support both reading and writing of ISO/IEC 29500 Strict. 

In 2000, Microsoft released an initial version of an XML-based format for Microsoft Excel, which was incorporated in Office XP. In 2002, a new file format for Microsoft Word followed. The Excel and Word formats—known as the Microsoft Office XML formats—were later incorporated into the 2003 release of Microsoft Office. 

Microsoft announced in November 2005 that it would co-sponsor standardization of the new version of their XML-based formats through Ecma International as "Office Open XML". The presentation was made to Ecma by Microsoft's Jean Paoli and Isabelle Valet-Harper. 

Microsoft submitted initial material to Ecma International Technical Committee TC45, where it was standardized to become ECMA-376, approved in December 2006.
when was the new format just for word released?
 In 2002, a new file format for Microsoft Word followed