Microsoft as changed its default file format in Office 2007. For example, .doc has become .docx and .xls has become .xlsx. The new formats now heavily depend on the XML file format. So, while you can open files of the older format using Office 2007 (in Compatibility mode), you won’t be able to open files saved in the new format with older versions of Office.
However, if you have any of the three prior versions of Office – 2000, XP, or 2003 you can install the Office compatibility pack and then you would be able to open the new format files with your existing Office installation.
You can download the Office Compatibility pack here.
Suggested Reading: Exploring the insides of a .docx word document