Ah, the Leaderboard – it is the younger, flashier cousin to the original 468×60 banner and is now widely used throughout the Internet. While the 728×90 – Leaderboard is one of the standard recommended AdSense ad sizes by Google, it is also a standard size recommended by Microsoft Advertising (Bing/Yahoo!) and the IAB (Interactive Advertising Bureau).
When used in AdSense, the 728 x 90 – Leaderboard ad unit can contain either text ads or image/rich media ads and is intended for the very top or bottom of a site and doesn’t usually fit in the constrained area of a blog content area.
An example of the text version of the AdSense Leaderboard can be found below. Leaderboards typically contain graphic banner ads and not text ads as shown in this example. However, if you click on the text (not the link) in the banner above and move your mouse to the right, the content area should scroll and you’ll be able to see the rest of the text that Google puts into the text version of the 728×90 Leaderboard in AdSense.