Microsoft Excel is the spreadsheet software included with Microsoft Office. Excel spreadsheets created with Office 2007 or newer use the file extension .XLSX instead of .XLS. If you just need to view or print an existing spreadsheet you can use the Excel Viewer. If you need to edit or create a new Excel spreadsheet you will need to use OpenOffice.org.
XLSX Excel Spreadsheets are different than the XLS version in that they use the Microsoft Office Open XML format. The Open XML format makes it easy for other programs, such as OpenOffice.org, to read XLSX files (and other Open XML file types). The Open XML format was introduced with Microsoft Office 2007.