Google Web History records your key search and web activity after you log in to Google Account. When you create a Google Account at http://www.google.com/accounts/NewAccount, you're given the option to enable Web History with your account. Visit Google Web History for more detail.
Many people choose to NOT enable Web History for privacy reason. However, when you activate Google+ service, you may accident enable Web History. For more information, read "Think You’ve Disabled Google’s Web History Tracking? Check Again" posted at TechCrunch.
To complete disable Google Web History, after login to Google Account, type this URL:
(Source: Alex at Google Web Search Community)