XLSM

XLSM is the variant of XLSX (the Microsoft Excel format used starting with Office 2007) which allows the use of macros. This poses a greater security risk than normal non-macro spreadsheet files, because macros are a form of computer program which can do actions automatically, possibly malicious ones, which is why a distinct extension is used to distinguish documents which may have macros in them from ones that don't.

Macros are created in Visual Basic for Applications (VBA).

Links

 * VBA tutorial
 * Introduction to Macros and VBA Code