Excel 2010 File Extensions
Excel 2010 supports various file types and is compatible with previous versions. Macros are not allowed to be stored in the default file type. You must use the Save As setting for all of your macro workbooks, or you can change the default file type used by Excel 2010.
Available files types are:
Excel Workbook (.xlsx)
Files are stored as a series of XML objects and then zipped into a single file. This new file-saving paradigm in Excel 2010 allows for significantly smaller file sizes. It also allows other applications (even Notepad!) to edit or create Excel workbooks. Unfortunately, macros cannot be stored in files with an .xlsx extension.
Excel Macro-Enabled Workbook (.xlsm)
This is similar to the default .xlsx format, except macros are allowed. Basically, the concept is that if you have an .xlsx file, you will not need to worry about malicious macros. However, if you see an .xlsm file, you should be concerned that there might be macros attached.
Excel Binary Workbook (.xlsb)
This is a binary format designed to handle the larger 1.1-million-row grid size in Excel 2010. Legacy versions of Excel store their files in a proprietary binary format. Binary formats might load quicker, but they are more prone to corruption, and a lost bits can destroy the whole file. Macros are allowed in this format.
Excel 97-2003 Workbook (.xls)
This format creates files that can be read by anyone using legacy versions of Excel. Macros are allowed in this binary format. Note that when you save in this format, you lose access to any cells outside of A1:IV65536. Additionally, if you open the file in Excel 2003, you will lose access to anything that use features introduced in Excel 2007 or later.