Personalized search reshapes your results primarily by noting the types of sites you select from the search results. That allows Google to look at those sites and then give them a boost in the rankings, especially if you visit them often. In addition, Google can determine sites and pages that seem related to those you are already visiting, in order to give them a boost.
Beyond your search history, Google also looks at the content on your Google Personalized Homepage - what gadgets you have there, feeds you are reading and so on -- in order to shape your personalized search results. This is a signal they've just started to use in 2005. Another new signal is Google Bookmarks. Pages you save in these also influence the results. In case you're wondering, your email in Gmail does NOT have an impact, as Google explains here.
To get personalized search in the past, you generally had to sign-up for the service specifically. Now everyone gets it. You even get it if you disable Search History. "We'll just have less to work with in terms of making your results better," said Sep Kamvar, engineering lead for personalization at Google.
Many queries aren't changed at all, Google said. For those that are, there are generally slight shifts in the rankings.
"What we've been doing before is taking two or three results that were suited to your tastes and injecting them. That's unchanged," said Marissa Mayer, vice president of search and user experience at Google.
I always joke that personalized search is also a good ego search reinforcer. People tend to go to their own sites often. That helps make your own site rank better in the results. This would happen to me with Google Personalized Search, and I'd get excited. Then I'd see the "Personalized Search" message and wonder if I'd still be there in regular results when I switched the results off.
This brings me to the search marketing aspect. It used to be only some searchers got personalized results at Google. Now everyone who is signed in to a Google service is going to get them, as I explained above.
This means the days of everyone seeing the same results for any particular query are growing more numbered. What's number one for one person might be number three for another and not even show up in the top ten for yet another.
