If you stay logged into anyone of the Google features like gmail, your search history will be accumulated by Google. So, if you start searching Google when you are logged in, google looks up your search history and your SERP (Search Engine Results Page) Results will differ. Google will show you the sites you liked most.

You can see "Your Search History" tab at the right side of Google search next to your email ID. You will find your past search history by using the calendar. If I search "Software Solutions" and click "iLink Systems" very often, I'll get iLink's web page at the top of search results. If you are not aware of personalized search, then you'll be getting the same repetition of sites in your SERP. Below are the Advantages and Disadvantages of Personalized Search.

Advantages:

• Get your most favorable search results
• It spreads traffic around different websites. So, the top 10 search results won't take all the traffic
• We can delete our search history

Disadvantages:

• We'll have to log out of the account if we need the actual search result or we can use two browsers like, checking google accounts in IE and SERP with Firefox.
• We can delete our search history but we can't delete it from Google database. Ie, After deleting the search history, we'll still get the same personalized search results.
• We'll get the smaller version of the web.

Google is now automatically turning on the personalized search without notifying the user. It can provide options to the user to turn it ON or OFF. If the Google personalized SERP is in the visitor's control (If we delete the history, Google should refresh our personalized SERP's), personalized search would also be a good feature of Google.