I want to create dynamic content based on this. I know it's somewhere, as web analytics engines can get this data to determine how people got to your site (referrer, search terms used, etc.), but I don't know how to get at it myself.
You can use the "referer" part of the request that the user sent to figure out what he searched for. Example from Google:
So you must search the string (in ASP(.NET) this can be found be looking in Request.Referer
) for "q=" and then URLDecode the contents.
Also, you should take a look at this article that talks more about referrers and also other methods to track your visitors: