.xls is a file extension used on computers running Microsoft Windows to identify files created in Microsoft Excel.
A file extension is the group of letters that appear after the last period in a file name. File extensions are usually 2 to 4 characters long.
.xls was the default file extension for versions of Excel up to Excel 2003.
With Excel 2007 Microsoft introduced a new default file extension ( .xlsx ).
The newer versions of Excel can save files in the .xls format in order to make them compatible with earlier versions of Excel.To do this use the Save As option and choose Excel 97-2003 Workbook *.xls) from the Save as type list in the dialog box.
For most users, the main benefit of the file extension is that it allows them to double click on .xls files and they will open in Excel.
File extensions can also make it easier to identify files when viewing them in My Documents or Windows Explorer.