Forum OpenACS Q&A: Where is the pieces of code?

I need to add metatags on my pages such as keywords, description and etc...
But i can't find where should i put them. and I know there is a place to put that the metatags will be placed on all the other pages as well.
So,
If i go to the browser and hit "View source code". I get the whole webiste on html code and can I easily see that the heeader is there.
But i don't have any idea from where and how the header comes to the page. If it's on the db, or a separate page included...
And i hope when i understand that issue i will understand how the system works. Well i have an idea but i didn't get at all.

Here it's a litle part of the header that i got when i hit on the browser "View Source Code"

!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">;
html
head
title /title
meta name="generator" content="OpenACS version 5.2.d9"
link rel="stylesheet" type="text/css" href="/resources/acs-developer-support/acs-developer-support.css" media="all"
link rel="stylesheet" type="text/css" href="/resources/acs-templating/lists.css" media="all" /
link rel="stylesheet" type="text/css" href="/resources/acs-templating/forms.css" media="all" />
link rel="stylesheet" type="text/css" href="/resources/acs-subsite/default-master.css" media="all" /
link rel="stylesheet" type="text/css" href="/resources/dotlrn/dotlrn-toolbar.css" media="all" /
script type="text/javascript" src="/resources/acs-subsite/core.js" language="javascript"/script