With AdSense for Games, advertisers can target campaigns based on keywords, genres or even specific games.

FORBES: Advertising

He simply instructs his readers to target Facebook ads by listing keywords, such as blogger, editor-n-chief, correspondent, news editor, writer, columnist, and dozens more in the interest field.

FORBES: How To Triple Your Success Using Social Media Advertising Platforms

ISP's network scans passing web pages for keywords that are then used to target ads.

ECONOMIST: Behavioural targeting