Workbooks can export data in a format compatible with Microsoft® Excel™. Note that it is not compatible with most other spreadsheet software such as Google Spreadsheet and the Open Office spreadsheet ('Calc') - for these applications please use the Export to CSV function instead.

The format which Workbooks generates is known as 'XML Spreadsheet'. Microsoft introduced this format in Excel 2002 and all modern versions of Excel can read it.  

This spreadsheet format has a particular benefit for large exports in that Workbooks can start to send data to your web browser before it has completed reading all the requested data.  For large exports this means your download starts much more quickly.

Excel does however present a warning message when you open this format (click to enlarge): 

Microsoft knowledge base article 948615 describes the warning message and how you can prevent it.