Ask HN: How do website analytics tools track where traffic came from?

I'm curious to know how something like Google Analytics can tell where traffic is coming from to your site? Do they just rely on document.referrer? or is there more happening under the hood?

For UXWizz I just use the Referrer header passed by the browser, it's not very reliable though. Sometimes it's very hard in analytics/business to tell "how did this user find me", the answer is very rarely simple (e.g. even if it's from a Google search, you don't know exactly what prompted the user to search for your business or what exactly they searched for).