Content
The content-based advertisements can be targeted for users with certain interest or contexts. The targeting can be CPC (cost per click) or CPM (cost per thousand impressions) based, the only significant difference in CPC and CPM is that with CPC targeting, earnings are based on clicks while CPM earnings recently are actually based not just per views/impression but on a larger scale, per thousand impression, therefore driving it from the market, which makes CPC ads more common.
There are various ad sizes available for content ads. The ads can be simple text, image, animated image, flash video, video, or rich media ads. At most ad sizes, users can change whether to show both text and multimedia ads or just one of them. As of November 2012, a grey arrow appears beneath AdSense text ads for easier identification. Google made a policy update regarding the number of ads per page, the three ads per page limit has been removed.
Search
AdSense for search allows publisher to display ads relating to search terms on their site and receive 51% of the revenue generated from those ads. AdSense custom search ads can be displayed either alongside the results from an AdSense Custom Search Engine or alongside internal search results through the use of Custom Search Ads. Custom Search Ads are only available to "white-listed" publishers. Although the revenue share from AdSense for Search (51%) is lower than from AdSense for Content (68%) higher returns can be achieved due to the potential for higher Click Through Rates.
Video
AdSense for video allows publishers with video content (e.g., video hosting websites) to generate revenue using ad placements from Google's extensive advertising network. The publisher is able to decide what type of ads are shown with their video inventory. Formats available include linear video ads (pre-roll or post-roll), overlay ads that display AdSense text and display ads over the video content, and the TrueView format.Publishers can also display companion ads - display ads that run alongside video content outside the player. AdSense for video is for publishers running video content within a player and not for YouTube publishers. it mostly shows in Youtube.
Link units
Link units are closely targeted to the interests of your users. Because users directly interact with the ad unit, they may be more interested in the ads they eventually see.
AdSense publishers are paid for clicks on the ads that are linked from link unit topics, not for clicks on the initial topics themselves. The ads on the linked page are pay-per-click Google ads similar to those shown in regular AdSense ad units. See more : Link Units
