XLS vs XLSX
XLS and XLSX are two file extensions that are used by the very popular spreadsheet application from Microsoft named Microsoft Excel. XLS is very popular as it has been the default format for Microsoft Excel since it was first created up till 2003. In the Microsoft Office 2007 release, Microsoft decided to change the default file format to a different format and adding and additional x for all their document extensions; for Excel, this ended up as XLSX.
As XLSX is a full departure from the older file format that is used in older Excel applications, it is not readable in versions of Excel prior to 2007. This incompatibility hampers the fast adoption of the new software and Microsoft quickly addressed this issue by releasing a patch that allows older Office applications to read the new XML based file formats. As always, backwards compatibility is always a priority for Office applications. Despite adopting and promoting the new XLSX format, newer versions of Excel are still able to open and save documents to the older XLS format. Although it is worth noting that Microsoft Excel 2007 has dropped support for older file formats that were commonly found in MS-DOS.
As we look deeper into the differences between XLS and XLSX, we can see that the way the information is stored is vastly different for both XLS and XLSX formats. XLS is based on BIFF (Binary Interchange File Format) and as such, the information is directly stored to a binary format. On the other hand, XLSX is based on the Office Open XML format, a file format that was derived from XML. The information in an XLSX file is stored in a text file that uses XML to define all its parameters.
As XLSX is stored in a text file format, Microsoft decided to remove macro support for this file format. Instead they assigned a totally different file extension that allows the use of macros; it is named XLSM. The older XLS file extension does not have this issue and it is able to hold spreadsheets that contain macros or not.
Summary:
XLS is the default file format for the 2003 version of Excel and older while XLSX for versions since 2007
XLS is readable by all Microsoft Excel versions while XLSX is only readable by versions 2007 and later
XLS is a proprietary binary format while XLSX is based on Office Open XML format
XLSX is not able to support macros while XLS is