Both XLS and XLSM files are spreadsheet files for Microsoft Excel, a widely-used spreadsheet program.The main difference between the file types is that XLSM files contain macros and XLS files do not.
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Macros are scripts that perform small, automated tasks within another program. Macros in XLSM files can help you complete repetitive tasks, like formatting columns of data, more quickly.
Microsoft Excel disables macros by default because macros can be used to distribute viruses, potentially harming your computer. Even if an Excel document contains macros, Excel will not execute them unless you save the file in XLSM format.
You can open XLS files in a variety of spreadsheet programs, including OpenOffice Calc, WordPerfect Office and Gnumeric. Since you can only open XLSM files in Excel or WordPerfect, saving them as XLS files instead will make it easier to access the files.