A .docx file is a Microsoft Word XML Format Document file, a kind of Microsoft Word document.
Starting in Microsoft Office 2007, .docx files became the default file type. You may have noticed when you open older .doc files made in Word 2003, that they opened in "Compatibility Mode." Though they are both Microsoft Word documents, they are different file formats.
What you need to know:
If you have an old version of Word (and cannot open .docx files) download Microsoft's compatibility software.
If you use Open Office, you still have the ability to open and use .docx files as well as the traditional format.
To save a .doc format:
Open the .docx in Word. Select Save As.
Choose Microsoft 97-2003 document.