g1smd - 6:50 pm on Dec 10, 2012 (gmt 0)
The frontpage extensions allowed frontpage to manipulate common elements on multiple pages at the same time. For instance, the navigation bar could be defined as a common piece of code and frontpage would drop it into every page. If you edited the navbar, frontpage would update every page of the site without you having to edit every page individually. The extensions also synchronised the local copy of the site on your PC with the remote copy of the site on the server. There were many other functions available, most of them proprietry.
Nowadays, you're much better off to have the navbar as a small piece of code in a separate file and then use the PHP "include" directive on each page where that code should show up. PHP also allows a lot more things than frontpage could ever do.
Once you're fluent with PHP, learn mySQL or a later variant, so you can then move onto full CMS systems with the page content stored in a database.