The idea is to identify reusable components of a page, make a page with just that component, then insert it into every page that uses it with an include. This way when that component changes all you have to do is make the it once and the update cascades through out the rest of the site automatically. Below is a diagram of a popular page layout scheme. Components in red may be candidates for includes. A corner logo can often change to blend in with a the background of a new design.
Headers can contain the most basic navigational elements of a site.
They can have links back to the front door, contact information, a search engine, or rotating adds. The footer can contain the same information as the header. This type of global navigation is easy to maintain with includes. Each of these components can be made individually, then put together like a puzzle on a template page. A template page is a skeleton for each page in your site.