DWTs use metadata to maintain the relationship between a file and the pages attached to the file.
If for some reason, in my case I had duplicated a Site Definition in the Site Manager and changed the URL (for a new version of the site, not in version control for the time being) updating the templates no longer updates the pages created from that template throughout the Site.
This is written with the assumption that you are working with your site on your hard drive and NOT editing the site live on the server. You should receive a notice of the number of files attached to the DWT and asked if you want to update them. You should then get a notice like the one shown below.
The question occasionally comes up in various Expression Web groups and lists over what to do when pages with a DWT attached do not update when changes are made to the DWT. If all pages have updated, the numbers should match.
Then you add editable regions so that you can change the content in pages attached to this DWT.