It's a new file format that the latest version of Microsoft Word uses for it's documents. It is not backwards compatible with previous versions of Word. You can, however, download an add-on for older versions of Word that allows you to view/open/etc the docx documents. You can get it for free from Microsoft here:
http://office.microsoft.com/en-us/wo...444731033.aspx
There are somethings in the docx file that you might not be able to edit in the older versions of Word even with the package installed.
This happens frequently at our office. As our clients seem to have the newest and bestest before we do, there are many a befuddled coworker wandering the halls wondering why they can't open a simple word document.