Google Now Personalizes Everyone’s Search Results
http://searchengineland.com/google-now-personalizes-everyones-search-results-31195

Beginning today, Google will now personalize the search results of anyone who uses its search engine, regardless of whether they’ve opted-in to a previously existing personalization feature. Searchers will have the ability to opt-out completely, and there are various protections designed to safeguard privacy. However, being opt-out rather than opt-in will likely raise some concerns. The company has an announcement here. Below, a deeper look. The short story is this. By watching what you click on in search results, Google can learn that you favor particular sites. For example, if you often search and click on links from Amazon that appear in Google’s results, over time, Google learns that you really like Amazon. In reaction, it gives Amazon a ranking boost. That means you start seeing more Amazon listings, perhaps for searches where Amazon wasn’t showing up before.

In particular, we now have two “flavors” of personalized search, or “Web History” as is the official Google name for it. There’s Signed-Out Web History and Signed-In Web History.