How do advertising networks show ads?

I'm curious, how do advertising networks show advertisements? Example, here is a link you would put on your site for an advertisement:

<script language='JavaScript' type='text/javascript' src='http://clicktorrent.info/phpAdsNew/adx.js'></script>

Code (markup):

If you open that, this is all there is:

function phpads_deliverActiveX(content)
{
	document.write(content);	
}

Code (markup):

so what type of code is embedded in the javascript file that doesn't show?

 

anyone???????

 

no one knows?

perhaps a better question is, what code can you embed in a javascript file (.js) that doesn't appear on the client-end and can interact with a javascript function?