Word Document (DOCX)
DOCX is the file format for Microsoft Office 2007 and later.
DOCX should not be confused with DOC, the format used by earlier versions of Microsoft Office.
Relationship to OOXML
For most purposes OOXML may be considered a subset of DOCX (DOCX contains additional features, like OLE serialization).
Documentation on OOXML may provide a guide to analysing a DOCX file.