Delivering ads based on a user's habits. If a customer registers with an e-commerce site to make a purchase, those sales along with the user's site navigation history are often stored and analyzed to make targeted offers the next time. Even without registration, the site might place that information in a cookie, which is saved on the customer's hard disk. When the user revisits the site, the cookie is sent to the Web server.
A more invasive method is for ISPs to perform deep packet inspection on their customer's traffic to determine the types of websites they visit. The data are sold to marketing and ad serving companies to deliver more personalized ads. See cookie
and deep packet inspection