DOCX

Office Open XML (OOXML) representation (.DOCX) is the default file format for documents created by Microsoft Word as of Word 2007. The format is based on XML component files in a container based on the ZIP format. It replaced the binary DOC format used in earlier Word versions, and comes in two flavours, 'strict' and 'transitional' (see below).

This format (or the XML components of it) has been referred to as WordProcessingML, a name also used for the standalone XML files the earlier Word 2003 was able to generate.

Graphical inserted elements may be stored in the form of DrawingML, embedded in the XML.

History
This (along with the other Office Open XML document types PPTX and XLSX) was initially standardized as ECMA-376 in 2006. Three versions of ECMA-376 have been produced; the second version corresponds to ISO/IEC 29500:2008, approved as an ISO/IEC standard in April 2008. Changes to the standard between 2008 and 2012 were primarily corrections based on individual defects reported as implementation of the standard proceeded and required to ensure functional interoperability with existing applications. They do not introduce new functionality.

High-level structure
Like the other "Open XML" formats, this file format actually consists of various files (mostly XML) compressed into a ZIP archive, with this fact obscured from the end user by the use of a different file extension.

Strict versus Transitional
The OOXML standard actually defines two different format variations: 'strict' and 'transitional' OOXML. The transitional form is not fully specified within the standard documentation, as it is very closely bound to the specific behaviour of Microsoft Office and the older binary formats. The strict form is the fully standardised form, but Microsoft have been slow to fully support OOXML-Strict as the default format for Office documents, leading to interoperability problems. See this blog post for a more detailed look at the interoperability issues, and here for some context from 2014 concerning government support for open formats. Some more commentary is here.

Specs

 * ECMA-376 specification
 * ISO publicly available standards, including the latest ISO/IEC 29500 specification (as of November 2012, this is ISO/IEC 29500:2012)
 * OOXML Format Family -- ISO/IEC 29500 and ECMA 376 (Library of Congress)
 * DOCX Transitional (Office Open XML), Library of Congress
 * DOCX Strict (Office Open XML), Library of Congress
 * Markup Compatibility and Extensibility (Office Open XML), Library of Congress

Sample files

 * Windows Word 2007 sample file
 * https://telparia.com/fileFormatSamples/document/docx/

Software

 * Pandoc: Document format conversion swiss-army knife
 * docx2pandoc: translate DOCX to Pandoc output formats
 * Textract: extract text from various document formats including DOCX
 * Python library for parsing Office Open XML files

Other links and references

 * Office Open XML : Wikipedia
 * How to open new file formats in earlier versions of Microsoft Office
 * Forensics Wiki article
 * Why it might be time to dump Word
 * Complex singularity versus openness