How to Target Sections of your Site for Google Adsense
Please note that this post is over a year old and may contain outdated information.

<!-- google_ad_section_start -->
<!-- google_ad_section_end -->
This is important to do, because other content on your site, such as in the header, side bar, or footer, may not be relevant at all to the ads you want displayed for a specific page or blog post.
By targeting just the relevant content of a specific page, you can make sure the Adsense ads that are displayed are as relevant as they can be, which will improve your "clicks" and hopefully annoy your visitors less.