XLX (and XLSX) are the file formats used by Microsoft Office's Excel spreadsheet program. These formats can also be opened by office programs on Ubuntu such as Openoffice or Libreoffice.

The XLS data is stored in the "Binary Interchange File Format" (BIFF) file format. Details of this format are given in Excel 97-2007 Binary File Format Specification.

The XLSX format is the Office Open XML (OOXML) format used by Excel 2007 and later.

The main advantage of the XLSX format over the XLS format is that it allows a larger number of rows and columns in a worksheet.