The term contextual advertising means the advertisements that you see on websites or in other Medias like in the displays of a mobile phone, where the advertisements are chosen and served by an automated system that is based on the content displayed by the user. For example, if you wanted to place a contextual ad on your website and your website is about books, then the advertiser will display ads about bookstore and books and anything book or magazine related. The ads will reflect the content of the site.
Since the concept of Adsense, began others have long since began their own AdSense like Yahoo! Publisher network, Microsoft ad Center and others have thrown their hats in the ring. The concept of contextual advertising has made a huge difference in the way webmasters make money on the Internet. The more closely the ads are related to the content the more likely it is that visitors to the site will most likely click on these ads which in turn makes money for the webmaster and for the server of the advertiser.
A big part of Google's revenue is from their potion of the contextual ads that are being used right now on millions and millions of WebPages in the AdSense program. Some of the other contextual advertisers are AdBrite, BidClix, Q-Ads, Kanoodle Bright Ads, Targetpoint, Searchfeed.com and more. Just sign up on for these AdSense companies and sign up and get started. Once you apply, it is not guaranteed what ads you are going to get, you really do not get to pick and choose but the ads that are appropriate for your site you will get. There are not any AdSense companies that will put ad that is not affiliated with your content.
The goal they understand is for you to make money because the more money you make the more money they make. They have your best interest at heart because when you make money they make money.