When you save a document for the first time or if you want to save a copy of a file,
use the Save As command.
When you want to save an open document,
use the Save button on the Quick Access Toolbar.
When you save a document, Word 2007 saves 97-2003 files in an older format using compatibility mode,
and saves new 2007 files in an XML (Extensible Markup Language) based file format.