Although Google holds some of your personal information, it also gives you the power to control how much information it can collect. There are many different privacy tools to help you modify your privacy settings, including the Ads Settings, Web History Controls, and Google Dashboard.
Many websites partner with Google to bring you more relevant ads. In order to do this, Google tries to guess your interests and demographics—including your age and gender—based on the websites you visit. When you perform a Google search or view a webpage, Google will use this information to decide which ads to show you. This is meant to benefit both you and advertisers: You're more likely to see ads that interest you, and advertisers know their ads are more likely to pay off.
However, not everyone likes ads based on their interests. If you would like to turn off ads based on your interests—or only see what information Google is using to deliver ads to you—you can go to Ads Settings.
Your interests and demographics are not stored in your Google account but rather in a cookie on your computer. This means you'll get interest-based ads even if you're not signed in to a Google account.
View the following video by Google for more information on how interest-based advertising works.
If you clear your browser's cookies, you will need to go back to Ads Settings and opt out again. You can also install the Keep My Opt-Outs plug-in for your browser, which will keep you opted out of Google ads permanently even if you clear your cookies.