Question about template systems

Hello, First post here, I'm in the process of learning PHP , I'm digesting a few books as we speak. I'm working on a content heavy website that provides a lot of information, a template system would be great and so i've been looking at ways to create dynamic data with a static navigation system. So far, using the require_once(); function seems to fit the bill in order to bring in the same header html file on each page. I've also looked at Smartys template system. I wondered how you folk would go about creating a template system ? My second question might be me jumping the gun here, I haven't come across this part in my book but i'll ask about it anyway. I often see websites that have a dynamic body and static header, and their web addresses end like this: "index.php?id=445" where 445 i presume is some file reference. What is this called ? It seems like the system i'm after but it doesn't appear in my book, If anyone could let me know what this page id subject is called i can do some research on the subject. Thanks for any help you can provide :) Matt.