Google self improves it’s search results using it’s Personalized Feature. It tracks your Google Searches and what links you click and according to that it shows results that match the results you clicked.

Till now the feature was available to only those users who had enabled web history and we signed in to their Google account. Now, it will track your searches for 180 days and refine your results according to the searches without the need of being signed in.

This could be another major improvement in Google Search which drastically improves search performance. It track web history via a Anonymous browser cookie.