Excel 2010 workbooks use the .xlsx file extension. Excel 2010 is based on Office Open XML formats (but you do not need to know how to use XML to work in Excel 2010).
The reasoning behind this change is because XML helps keep workbooks safer by
You can open workbooks that were created in previous versions of Excel. When you open such a workbook, it will open in Compatibility Mode.
When you save a file that was created originally in a preview version of Excel, the Save As dialog box will open automatically, to save the file with an .xlsx file extension.
Also, when you save an Excel 2010 file as a previous version, Compatibility Checker will identify any new features that will not work in that version.