The file extension XLS is a proprietary file format of the Microsoft Excel spreadsheet program. The .xls format is the standard format for versions 97 through 2003. As of version 2007, Microsoft has released a new version of the Excel Spreadsheet format.
An XLS file contains table cell values as well as formatting instructions, formulas, and possibly macros. These macros may also contain malicious code, which is why it is advisable, especially for XLS files from unknown sources, to subject them to virus scanning before executing the macros.
XLS is based on the proprietary binary format of Microsoft, the Binary Interchange File Format (BIFF) format, which, however, can still be read by almost any spreadsheet program due to its age and its high popularity.