The term contextual ads means the ads you see on webpages or other media, such as on mobile phone displays, where the ads are selected and served by automated systems based on content displayed by the user. For example, if you want to put a contextual ad on your website and your site is about books, advertisers advertise ads on bookshops and books, and any book or magazine related. The ads will reflect the content of your site.
Google AdSense was the first largest contextual advertising program that became accessible to users. These ads are working by giving webmaster JavaScript code as when you insert the code into a website you want to advertise on it will draw similar ads from the Google advertisement of advertisers. The way the relevance of the ads related to content is calculated by what Google Bot has indexed.
Since the term Adsense, others started long ago, starting their own AdSense like Yahoo! Publisher networks, Microsoft Advertising Center and others have thrown their hat in the ring. The concept of contextual advertising has been very important in how webmasters make money online. The better the ads relate to content, the more likely it is that visitors to the site most likely click on these ads, which then generate money for webmasters and advertiser server.
A large part of Google's revenue is from their dear contextual ads currently used on millions and millions of websites in the AdSense program. Some of the contextual ads are AdBrite, BidClix, Q-Ads, Kanoodle Bright Ads, Goals, Searchfeed.com and more. Just sign up for this AdSense company and sign up and start. Once you apply, it is not guaranteed which ads you are going to get; you really do not get to choose and choose the ads that are relevant to your site you will receive. There are not any AdSense companies that want to put an ad that is not affiliated with your content.
The goal they understand is that you should make money because the more money you make them the more money they make. They are interested in your heart because when you buy money, they are money.