An Ad unit is the set of ad which is displayed as a result of the code which you get from the Google adsense. There are different ad size and different format of ads which can be displayed on the blog or a website.
But what kind of ad unit is the best for Google adsense earning. According to Google adsense wider the ad unit, better it is. It allows publisher to put a wide image or text for display. Readers are more likely to click ads which is wider and has both image and text format. It increase advertisers for more bidding to put ads on the blog.
So what type of ads are better. According to Google adsense, “728 x 90 leader board” is best to display at the header of the webpage.
“300 x 250 rectangle” is best to be displayed in the content.
“336 x 280 rectangle” can be placed on the right side bar at the top.
“160 x 600 skycrapper” is best to be displayed on the right sidebar at the bottom of the page.
Experiments have shown that this produces higher CTR (Click through Rate).
But we should remember that all of these customization only works when our blog or website has decent amount of Traffic and the quality of the content is good.