Google currently offers : AdSense for Content, AdSense for Search, and Youtube Adsense
AdSense for Content
AdSense for Content is the main AdSense program - contextually targeted ads are displayed on your webpage, and you get paid for every click. These ads are specifically targeted to the content of your webpage. Consequently, if you change the content of your page, the ads that are displayed would also change. AdSense allows you good control over what ads get served, since there might be some obvious problems if it didn't. These include:
Blocking Ads - you choose
While the ad selection is automatic, you can block unwanted ads - such as those from your competitors. You just tell Google which companies to block from your sites.
Reviewing and filtering Ads - semi-automatic
Google has its own strict policies for ad content (no profanity, no racial discrimination, etc.). These are filtered out automatically from your point of view. Additionally, you can choose to block additional content through custom filters. This is slightly different than blocking an advertiser, since with this method an advertiser you generally allow may submit ads that would individually be filtered out.
Allowing the publisher to select an ad
In the rare case where Google's content engines can't make a clear match of ads to pages, they will let you pick which ads you want served. If you don't pick, you'll get public service spots.
The look-and-feel of Ads can be customized
Ads shouldn't look out of place or be jarring on your website. Google allows you to customize colors and layout. You can choose options provided or have your own custom layout and color applied to the ads. The advertisers have no control over this; as the material appears on your site, you can and should select how it looks.