(You won't see a change if you click the link to the version of the site you're currently on.)
Same content accessed through a different HTML page, constructed in a different way. The link above simply points your browser to the alternative HTML file. You could use a similar approach to redirect users to a low-bandwidth or mobile site. The alternative site accesses the same content directory but doesn't otherwise interact with the main site.
While the original template uses new HTML standards and CSS for a fluid site layout, the 90s template uses an old-fashioned layout method based on HTML tables. FolderCMS is a simple tool that doesn't dictate web design policies.
The site content all comes from the same content directory, but not all pages have to be the same. Navigate to the home page for an example of alternate page content specific to this demo. (Auth flags are used to achieve this.)
As you navigate between the top level headings, not that the colour of the left-hand menu changes. This is made possible by the cascading templates feature, which allows you to apply almost anything to an entire subcategory of content.