In the old versions of Excel, up to 2003, workbooks were named .xls and macros were stored directly in the workbooks. But with increasing concern about malicious software, Microsoft introduced a distinction between workbooks that can contain VBA code and those that can't. The plain-vanilla workbook is now .xlsx. If you want to write a macro for one of your workbooks, it won't be saved in an .xlsx; you have to store it with extension .xlsm. This provides a little extra warning about the content to a user opening a workbook, in addition to the warnings that pop up.

I graduated straight from 2003 to 2010, so I don't know whether this distinction started with 2007 or 2010.

Does that help? If I skipped over an important part of the explanation, feel free to keep asking.