Hi,
the system is based on a simple include() instruction.
When you look an url like this main.php?id=reviews probably main.php has a core like this
include($id.".php");
This way you can store your code like a frame system, but without using the frame.
For example, in main.php, you can store the top banner code, with logo, usefull link, and some other stuff that are the same for all the pages. You can also put in main.php the left row with the menu. You will need only to load the core of the page, the central table that is different for each page.
This give you a nice maintainability (you can modify the menu for all the pages just modifying the menu in main.php...) and is more compatible with browser than frames.
But is not all gold...
First, the cache system: even if the gfx is the same for all the page, probably each page download a new copy.
Second, hacker: this system is very very opened to hacker attack like XSS (I've read some good guide even on astalavista...).
A secure way to do it is a good control of $id var. Any secure way is to lookup the page in an array and to use a numerical id:
$lookup[0]="home";
$lookup[]="reviews";
$lookup[]="tool";
...
include($lookup[$id].".php");
And calling it with main.php?id=1
I hope this helps
bye bye