what is the difference between an excel file with the extension .xlsx and .xlsm ?

Xlsx: This is the XML-based default file format for Excel 2007 / 10.
This file format cannot store Ms Visual Basic for Applications (VBA) macrocode or Ms Office Excel version 4.0 macro sheets (.xlm).
Xlsm: This is the file format based on XML and macro-enabled for Excel 2007 / 10.
This file format Stores VBA macrocode or Excel version 4.0 macro sheets (.xlm).
Xls: This file format is used in earlier version of Excel 97 – 2003 and known as Binary file format (BIFF8).

an excel file with the format xlsm contains macros and the xlsx one does not contains any macros