XLSM, Excel Open XML Macro-Enabled Spreadsheet (.xlsm)
The XLSM Spreadsheet file format
XLSM is the file extension that is assigned to spreadsheets created by Microsoft Excel 2007 and newer. They are saved using the Open XML standard. Just like older version of the Excel spreadsheet, XLSM files can contain tables and worksheets. On top, XLSM files are macro-enabled. They can contain embedded macros that use VBA.
Technical details of XLSM files
As stated above, XLSM files are macro-enabled. This distinguishes the format from XLSX files that are also created by Microsoft Excel. Thus, for Excel to be able to execute the macros embedded in the file, the spreadsheet has to be saved in the XSLM format rather than as a XLSX file. Predecessors of Microsoft Excel 2007 are only able to open XLSM files if they support Open XML components.