Identification

• The file extension XLS is part of a proprietary format called Binary Interchange File Format (BIFF), which is used primarily as a spreadsheet extension for Microsoft Excel before 2007. In 2007, the Microsoft Excel format structure changed to XLSX.
  
  

Function

• XLS files store data within rows and columns in a spreadsheet. In most cases, XLS files are used to store numerical data, but graphs and formulas can be stored, too. These files are commonly used in accounting and scientific programs.
  
  

Warning

• Scan all XLS files before downloading and opening, as they can contain viruses and malware. There are various XLS scanning tools available online (see Resources).