When a person has viewed a Web page for a particular device several times, for example, Tapad considers that a sign of purchasing intent.

WSJ: Online Ads Can Now Follow You Home

Google recognizes the Web's power to publicize and offers Web masters a simple interface to remove their own pages from its index. (Type "remove" into Google to get started.) Other preventive measures include putting sensitive info behind password-protected walls and attaching so-called robot files to Web sites that tell search engines not to index a particular page or site.

FORBES: Google Me Not