An Iframe is like a little box on your page that all your links appears in. Then you don't have to make like a layout on every page, just make the pages you want inside the Iframe. If you still don't know what it is check it out on my frontpage, I use it there and I really like it :)

1. To use a iframe is really easy. First just copy and paste the code below on your page.
<iframe name="Iframe" src="URL" FRAMEBORDER="0" style="width:400; border:0; background-color:#000000; height:200; overflow:auto;"></iframe>

2. Where it says URL in the code you paste the url of the page you want to show when no links have been clicked ... To say it like this, the site that will show when you enter the page.
Change the size of the Iframe by changing the width and height in the code.

LINKS

To make the pages that belongs to your link appear inside the Iframe you will need to add target="Iframe" inside your link.
Example:
<a href="URL" target="Iframe">Link Name</a>
If you use that code your site will appear inside the Iframe when you have clicked it.