Implementing web analytics tags

I'm implementing my own primitive web analytics package. I'd like to add
javascript tags to pages so I can track usage. Most of the packages out
there ask you to include something like this in your pages:

<script
src="http://www.google-analytics.com/urchin.js"
type="text/javascript">
</script>
<script
type="text/javascript">
_uacct = "my_account_num"; urchinTracker();
</script>

(This is an example from Google Analytics)

I can't make heads or tails of the javascript in urchin.js, though. Does
anyone know what it is doing, exactly?

My guess is that it is capturing the URL of the page, and then issuing a
fetch to another page that passes that URL on, but I'm not sure how to
do that in Javascript.