With the release of Office 2007, Microsoft have taken the opportunity to overhaul the file formats for Word, Excel, and PowerPoint.
The new file formats are known as the Microsoft Office Open XML file formats, and are themselves derivatives of the XML (Extensible Markup Language) file formats commonly used in Web applications.
The .doc, .xls and .ppt file extensions are now .docx, .xlsx and .pptx, and also have a slightly different extension again if the file contains macros.
The move to the new format has two distinct advantages for the average user.
Ostensibly, files created by Microsoft Access remain unchanged from previous versions of Office
These are the standard Office 2007 file formats, by using the 'Save As' command it is possible to save your work in a variety of other formats including .pdf. (You will need to download the free plug-in from Microsoft for this)
It is possible in most of the programs to open files created in older versions of Office, and in a new departure Microsoft have made it easy to open documents created with Office 2007 in previous versions, although you need to download the Microsoft Office Compatibility pack from their website to do so.