Google Adsense is primarily a Cost –per –Click (CPC) Ad network. Google has another service called Google Adwords from where it gets advertisers. These advertisements are displayed on Google Adsense publisher's websites in the form of Text or Image Ads and the publisher is paid for every visitor click that happens through the publishers blog or website.
Google Adsense is also a contextual ad network. Contextual ad networks are those ad networks which display ads relevant to the content on the webpage. This creates a win-win situation for both, the advertisers as well as the publishers as it helps the advertisers to get better Return on investment (ROI) as the ads are viewed by people who are interested in the particular product/service whereas for publishers it means that since the ads displayed are related are to the content, it has more chances of being clicked by the visitor and thus generating revenue for the publisher.
Google Adsense accepts only the websites and blogs which have quality and unique content. If you are looking forward to signup with Google Adsense as a publisher, then make sure you follow all the terms and conditions which are specified.