Way back in the month of June 28. 2009, Google launched Personalized Search, so that the users can use that search history to get better results. As the search history grows you will be able to notice the difference. To do all that you require to get signed in to your Google account.
Personalized SearchJust on the same grounds now Google has added another functionality, which provides better search results by extending the personalized search for everyone, which includes the signed-out users too, that too in more than 40 languages.
Now when you search using Google, we will be able to better provide you with the most relevant results possible.
The personalized search option, was initially available to the users who had signed in to their Google account and had the Web History enabled. But from today the Personalized Search option is available to the signed out users also, and that too in 40 different languages.
The Personalized Search Theory
The results are customized based on the web search history for 180 days. The point to be noted here and not to be confused here is that it's completely separate from your Google Account and Web History (which are only available to signed-in users).
You have the option of viewing the customize search results in the "View customizations" link will appear on the top right of the search results page.
If you click the link, you will be able to see the customized results by Google and any time you can turn off this type of customization.