In early versions of Microsoft Word, documents were saved with the file extension "doc." Microsoft changed to saving documents with the file extension "docx" by default starting with Microsoft Office 2007 applications.
History
Microsoft changed the file-saving formats and file extension naming in its Office suite with the 2007 version. In Microsoft Word 2007, documents previously saved with the file extension "doc" now by default are saved with "docx" or "docm" file extensions.
Meaning
Microsoft added an "x" or an "m" to the file extension to indicate whether the file contains macros. Therefore, a "docx" file extension indicates a Word document without macros; a "docm" extension indicates a Word document with macros.
Compatibility
Users who want to send a Word document to other users can maximize compatibility by saving the document with a "doc" file extension rather than "docx." Users of Word versions prior to Word 2007 cannot open a "docx" document.