As like Google search results pages, Google is using it’s automated technology to scan the content (content extraction) of incoming e-mail for keywords and place related text ads inside the Gmail. For example if your sent you an email related to computer software, email automated technology will display ads in a fraction of a second related to computer software.
Google’s Gmail service, is part of the content network displays ads via its advertising program also known as Google AdWords (for advertisers).
Since Gmail is scanning and extracting incoming e-mails, privacy concerns raking up widely. According to Google, If their automatic filters detect that the topic of the email is sensitive, no ads are shown.
Google will display ads on Google and the Google Network sites. This includes Google search results pages, Google Product Search, Google Groups, Gmail and it’s extensive content network of websites and products.