Advertisers use placement targeting to display ads on specific sites and pages in the AdSense network. Placement-targeted ads reach users visiting specific sites, independent of their interests or demographics.
Interest-based advertising, on the other hand, reaches users based on the sites they've browsed and the interest and demographic categories associated with them. Interest-based ads are shown to users based on their inferred interests and demographics, rather than at a specific site or page.
For example, an advertiser selling footballs could reach customers through both types of advertising. They could use placement-targeted advertising to display their ads on specific football-related sites, and interest-based advertising to display ads to users in the sports interest category on sites without content specific to football.