Worksheets do not have extensions. They are contained in files known as workbooks. A 2007 workbook extension is .xlsx.
There are different names: Workbooks or Spreadsheets or Worksheets. Workbooks contain worksheets, and they can be called spreadsheets. They will have a .xls extension.
xlsx is the main extension used for Excel 2007.
For a standard Excel document, prior to Excel 2007, it is xls and xlsx is used in 2007. There are other kinds of files created by Excel, like templates which can be xlt or macros which can be xlm and xla for Add-ins, and there are other extenions. For 2007, you add x to the end of them.
Excel 2007 does use the .xlsx extension for its workbooks.
It is a collection of worksheets and known as a workbook, and has a .xls extension.
Worksheets are stored in a workbook in Excel.
Normally xls is the extension for an Excel spreadsheet document.
The standard extension for Excel workbooks is xls for versions up to 2003. Excel 2007 uses xlsx as its extension for workbooks.
A workbook in Excel is a collection of worksheets. You can tell if you have more than one worksheet in the workbook if you see multiple tabs at the bottom of the screen. The default setting of Excel 2007 includes three worksheets in the workbook, labeled Sheet 1, Sheet 2, and Sheet 3.
Those numbers are for each worksheet. You can have as many worksheets in an Excel 2007 workbook as the memory in your computer will allow. Excel 2003 and earlier are limited to 255 worksheets per workbook.
the normal extension for Excel is xls if pre 2007 otherwise xlsx
Excel 2003 and earlier: .XLS Excel 2007 and later: .XLSX