Personalized results were previously served only to users who were signed into their Google account and had opted in to let Google track their Web History, or log of search queries and results. Going forward, personalized results will be offered to users whether they are signed in or not. Users must also now opt out of personalized results.
This is how it works. Google will continue to use Web History to personalize results for users who are signed in. Even when users are not signed in, Google will customize their search results based on past search information linked to a user’s computer Web browser using an anonymous cookie. Google stores up to 180 days of signed-out search activity linked to the browser’s cookie, including queries and results that are clicked.