Previous Query refinement is now coming to unpaid or "organic" search results, [Google VP Marissa Mayer] said.This is a cheap and easy form of personalized search and personalized advertising that uses the history stored in the referrer instead of maintaining a separate, longer history.
For example, if someone were to search for [spain] and then [travel] after that, BOTH the ads and the organic results will be altered to take the previous query into account. To some degree, it will be as if the second query was for [spain travel].
This is a big deal. Big deal. It means that the results for many "single word" queries, which can be hard for sites to rank for when billions of listings come back, will become queries involving two or more words -- and much more specific ones.
As Danny says, the goal of personalized search is to help us find what we need faster and more easily by creating a dialogue with our search engine. No longer is each search treated as independent, but what we find or do not find in earlier searches impacts our future searches.