Since Excel 2007, Microsoft has split the classical .xls format to several formats (in particular, .xlsx, .xlsm, .xlsb).
.xls
.xlsx
.xlsm
.xlsb
One could think that xlsb has only advantages over xlsm. The fact that xlsm is XML-based and xlsb is binary is that when workbook corruption occurs, you have better chances to repair a xlsm than a xlsb.