Here I will explain about the difference between contextual advertising, placement-targets advertising and Interest-based advertising.
Contextual advertising: A contextual advertising uses advanced algorithms to analyze content of the page and shows advertisements based on what the user is viewing. For example, if the user is viewing a site about technology which uses contextual advertising, user might see ads related to technology.
Till now Google Adsense was using Placement-targeted ads, now it has launched interest-based Ads (beta) in it’s network. These ads will associate categories of your interest — say Technology, Sports. These interest are based on the types of sites you visit and the pages you view, while placement-targeted ads reach users visiting specific sites, independent of their interests. Check out difference between Placement-targeted ads and Interest-based ads.