Google has a new way of exploiting your privacy. It picks out the interests that are expressed in your email correspondence and charges advertisers to serve ads to you based on interests you express in them. Fortunately, you can opt-out of this intrusion by clicking the off button at https://www.google.com/settings/ads.
Here are step-by-step instructions:
Log into your Google account.
Visit https://www.google.com/settings/ads.
Look for the toggle button next to https://www.google.com/settings/ads.
Turn it off.