XLSM
From Just Solve the File Format Problem
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).
