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 users 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.