A .docx file is a type of document created in a Microsoft Word 2007 or later. The "x" stands for XML, the name of the new type of file format used by Microsoft Office applications after 2007, as opposed to the earlier .doc files, which used a binary format.
The .docx file is compatible with all Microsoft Word versions 2007 and later and can be used in previous versions with the help of a file converter. Versions of Microsoft Word 2007 and later also allow users to optionally save documents in the previous binary file formats to maintain compatibility.
XML file types are beneficial in that they offer better security, better file recovery and smaller file sizes.
A number of free alternatives to Word exist that have the ability to open and edit DOCX files. These alternatives include OpenOffice Writer and Google Docs.