Google Ad-Copy and Relevance

I've heard that Google ad position is, among other things, influenced by the relevance of the ad's target page/url.
Does anyone have an idea about how this works?

Is it based on an evaluation of page content from information already in Google's Natural Search db?
Is it based on an scan of the ad's page/url when the ad it posted to Google?

Thoughts?

Google uses a Quality Score based on a few things; https://adwords.google.com/support/bin/answer.py?answer=21388

Thanks for the reference Organic SEO!

Organic I do have a ?,
I understand that the Quality score is determined by:
the following
"This is the basis for measuring the quality of your keyword and determining your minimum bid. Quality Score is determined by your keyword's clickthrough rate (CTR), relevance of your ad text, historical keyword performance, the quality of your ad's landing page, and other relevancy factors."

but in regards to:
finding the relevance, this does Google scan over the page/s every time the ad is clicked, to find relevance on the landing page, or does Google have pre-recorded data for landing page relevance?

Does the relevance occur when you submit your ad's?
when you pages are submitted for your site map?
Or each time the ad is clicked?

thanks,