Microsoft went to the Open XML Format for its Office documents beginning in 2007. The Open XML format files are smaller than previous file formats. An Excel Workbook 2007 or 2010 business file ...

XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save workbooks, but can also use the XLS format.