How is it that advertisers are able to target your interest?

[KayKay]

[Jake]

Old School:
Magazine subscription and product warranty card information is rented to marketers for demographic targeting:
see lists.nextmark.com

Search Engine Keyword ads: Google Adwords being the best known shows "sponsored search" ads targeting specific search phrases, these can be some of the best ads offering a solution matched to an active information search. Google also scans the content of Gmail mail, showing ads on the email page matched to your mail content.

Internet banner ads: ad space on pages, particularly unsold "remnant" space is sold by trading desks in real time bids, your IP address and browser cookies are shared with prospective advertisers, an ecommerce site you recently visited may show you a "come back" advertisement, triggered by a cookie set when you visited their site.

Facebook paid ads: Have uniquely detailed user profile information that may be used for demographic ad targeting, interested you have demonstrated may also be targeted.

[Ralph]

They do a survey in the market to know what is trending. Then they make advertisements according to the taste of consumer.