Google currently offers four programs: AdSense for content, AdSense for search, adsense for domain and Premium 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.

Tools for analysis

Tracking and analyzing the performance of any marketing campaign is critical. Google AdSense provides a range of comprehensive online reports that allow you to monitor and analyze your earnings as well as the performance of ads by
many metrics, including size, color, and so forth, so you can make adjustments to maximize the value of the placements.

AdSense for Search

This is another program whereby Google allows you to ad a Google Search box to your website. By doing this, visitors to your site can search the entire internet from your website itself. In a way, your site becomes a host to Google. Except you make money!
The search results page that is displayed when a visitor on your website searches through the search box on the site itself, also displays Google Ads (AdWords ads) next to the usual listings, just as it would on Google.com. If the visitor clicks on any of these Google Ads, you get paid as well. Thus, with AdSense for search you can enhance your income potential beyond what just the content of your actual site can provide.
AdSense for search offers all the same options and controls as AdSense for content. These include ad customization, ad filtering, monitoring and tracking results, and so on.

Google AdSense Premium service

The AdSense Premium service provides advanced functionality and features, provided your website qualifies for it – which in fact very few sites do.
As a base requirement, your website should received at least 5 million search queries or 20 million page views a month to qualify for the Premium service. (We’re betting if your sites qualify for this level of service, you already know all about AdSense!)