I think I know what you're after, they're called frames and it means you navigation and header would remain the same and your content would open in the main area. This is generally frowned upon by web designers as it can cause search engines to ignore your content when spidering your site. But if that's what you want to do google frames and it should give you a little more info.
Here's a little insight
http://www.w3.org/TR/html401/present/frames.html