My title is not the best description of what I am asking but I am a user of Adwords and Adsense so I can see how this pans out. Google must "tag" my site with certain keywords and the Adwords ads it displays will be relevent to my site. I have made little changes to my site but nothing in they way of my core keywords. However the ads that are displaying are gravely different from what they were at first. How do I know how Google views my site in terms of keywords?
You don't know. The thing with google adsense is you accept which ads google puts up and either be happy with what you make with it, or try some other way of monetizing. (of course you should optimize, but that wasn't your question). The ads can change for all kinds of reasons - the main ones being: - google knows your content better, and has tracked which ads perform and which don't. - the advertisers themselves change. Some may have banned your site from their network, have decided not to advertise on the content-network at all etc.
You can use the tags for adsense keywords. 2 tags for your site. One tells adsense not to use the words in the tag. The other tells them to show ads for keywords in that tag.