Word processing software saves your documents in a wide variety of formats. Some of these formats become obsolete, and new ones are embraced. A file with the .docx extension is a new format for Microsoft Word documents.
The .docx format is part of the Office Open XML standard that was made official at the end of 2006. It was developed by Microsoft in cooperation with several standards bodies.
The .docx format is based on XML, rather than Microsoft's old .doc format. The main advantages of XML are compatibility with different programs and a much smaller file size.
The Office Open XML Family
The .docx format is part of a family of XML-based formats. Microsoft Excel has added .xlsx, and PowerPoint has added .pptx. These programs can still open the older versions of their files (.xls and .ppt).
Since this is an "open" format, a .docx file can be opened in non-Microsoft applications, and documents can be saved as .docx in non-Microsoft applications, without having to make legal negotiations with Microsoft.
These .docx files may not be viewable in older versions of Microsoft Office or OpenOffice. MS Office users may need to download a free update, which is provided in the Resources section