.xls for versions up to Excel 2003 and .xlsx from Excel 2007 onwards.
The standard extension for Excel workbooks is xls for versions up to 2003. Excel 2007 uses xlsx as its extension for workbooks.
The file extension of MS Excel is .xmls
.xls for 97-2003 and .xlsx for 2007 and 2010.
Version 2003 - .doc , version 2007 - .docx
Up to Excel 2003 normal Excel workbook files have the .xls extension. Template files have an .xlt extension. There are a few other types of files with .xl* where "*" is some other letter. From version 2007, .xlsx has been used.
XLS is the shorthand for Microsoft Excel file extension. A file extension is the group of letters following a period in a file name, denoting what kind of file it is.
Microsoft Excel is a spreadsheet.
Microsoft Excel is part of Microsoft Office. If that's what you mean.
XLS is the main extension name used in Excel.
It will depend on the specific type of file and version of Excel. The standard spreadsheet uses xls for versions up to 2003 and xlsx from version 2007. You can also have other types of files, like Excel template files, which would be xlt, or xlm for Excel macro files. There are several other types too.
.xls or .xlsx