Files with the .XLS extension contain a spreadsheet encoded in binary form, namely cells distributed into rows and columns and containing data, graphs, and macros. The files were unique to the Microsoft Excel software before the Excel 2007 version, in which they were replaced by the more universal and open XML-based format, XLSX. The XLS format is also supported by other programs for creating spreadsheets, such as OpenOffice Calc and Apple Numbers – however, it is Excel that is the most popular software of this type in the world, used not only in business and academical environments, but also for personal purposes.

Spreadsheets are used for organizing, analyzing, transforming, and picturing data. The current default format for spreadsheets is XLSX, and the default name file in Excel is Workbook1.xlsx, however apps still fully support the binary XLS format.