what is xls and xlsx
XLS is the file extension of the Microsoft Excel spreadsheet. It is created in the versions of Excel prior to 2007. It is a binary format based upon Binary Interchange File Format (BIFF) and thus it stores information in binary format. It is capable of holding the spreadsheets either containing macros or not.
File extensions are related to the file format, which is a computer programming term that specifies how information is coded for storage in a computer file. In the case of Excel, the current default file extension is XLSX and has been since Excel 2007. Prior to that, the default file extension was XLS. The difference between the two - besides the addition of the second X - is that XLSX is a XML-based open file format, while XLS is a proprietary Microsoft format.