Link units and themed ad units are two variations of AdSense Ads that are meant to further increase the quality of AdSense Ads that get served on websites.

Let’s take a quick look to see what ‘Link units’ and ‘themed Ad units’ actually are:

Themed ad units: As their name suggests, themed ad units are the ad units that
are based on particular themes. These themes could be depicted by colors or special
type of graphics that are representative of the event or occasion portrayed by the theme. Chad Timothy

The events and occasions could be things like Christmas, New Year’s Day, etc. Of course, the themed Ads can only be displayed if they are available from the advertisers and if you have enabled the themed Ads on your AdSense account (in ‘Ad Type Preferences’). Also, since the
events vary across different locations, the themed Ads are displayed based on the location of the website visitor. Besides this minor difference, themed ad units work in the same way as the normal ad units.
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Link units: Link units are another useful way of advertising through AdSense program. Though the basic premise remains the same (i.e. targeted advertising), the way link units operate is slightly different.

Link units are meant to provide even more meaningful advertisements to the website visitors by presenting them with a list of topics (ad keywords) to choose from. The website visitors are then presented with only those Ads that are related to the topic selected by them; hence making
their website visit more meaningful.

The list of topics presented to the website visitor is based on the website content. So, the topics are relevant to the context of website. Software Billions Club review

However, the ‘clicks’ on the topics doesn’t make any revenue for you; it’s the clicks on the Ads for that topic that earns you revenue. As far as implementing of Link units is concerned, it’s done in the same way as other AdSense type products — you copy and paste AdSense code in the html code of your webpage. Note that you
are allowed to implement only one link unit per web page.

AdSense related terms that you should know

Here is the list of AdSense related terms that you should know for sure:

Page impression: A page impression is generally the same as a page view but the number of page impressions can sometimes be different than the page views. This happens when the page is viewed by a visitor but the AdSense code is not executed for some reason (which means that the Ads are not displayed to the visitor).

Ad unit impressions: Whenever you get page impressions, you also get Ad unit impressions. However, the ad unit impressions are dependent on the number of ad units you have on your page i.e. number of ad units that get executed when the page loads on user’s browser. Note that the link unit impressions are also counted as ad unit impressions.

Click: A click (for the purpose of AdSense) is recorded whenever a website visitor clicks on one of the AdSense Ads. Note that a click on a topic in a link unit is not considered as a click.

Click through rate, for the purpose of AdSense, is the number of clicks received by an ad unit divided by the total number of ad unit impressions for that ad unit. So, a better CTR can mean more revenue for you.

Effective CPM: It is calculated as the cost per 1000 impressions. However, do not confuse effective CPM with your earnings. In fact, effective CPM is calculated by dividing your earnings by the number of page impressions and then multiplying the final figure by 1000. Effective CPM is generally used to compare various channels and determine which channels are more profitable for you.