Office Open XML
From Just Solve the File Format Problem
Office Open XML (not to be confused with OpenOffice XML) is the collective name for the XML-based formats used in versions of Microsoft Office since 2007, and also made into a formal standard for use by other vendors, though it's a really convoluted standard due to the need to support all sorts of legacy features of Microsoft products which were originally implemented in proprietary formats. The formats consist of XML data (and other related data files) compressed in a ZIP archive, renamed to a different extension based on what sort of document it is.
These formats include:
