When Microsoft introduced Office 2007, they switched the primary Word document format from .doc to .docx. For various reasons, this Office Open XML format is generally a much better storage format, but it anyone without Office 2007 will lack the appropriate tools to open the file.
You have two primary options if you wish to convert a .docx file to .doc, so that you can view the contents.
If you already have an older version of Microsoft Office you can install the Office Compatibility Pack, which adds .docx open/edit/save functionality to Office 2000/2003. This download is 27.5MB in size and means you can quickly upgrade your existing Office installation to cater for .docx files.
If you don't have Microsoft Office you can open the files using Microsoft Word Viewer, or use some of the online converters to turn docx files into doc: