After buying Blogger, Google launched their 'Content-Targeted AdWords' program.

Google would insert their AdWords on the weblogs hosted at Blogger and on any other site that wants this.

To do this Google uses a script that looks something like this one:

<script language="javascript"
src="http://pagead.googlesyndication.com/ca/show_ads.js"></script>

If you analyze the script of
http://pagead.googlesyndication.com/ca/show_ads.js , you notice that pagead.googlesyndication.com server (created by Google) is called from the page that is loaded into the browser.

This server returns a piece of code generating a 'banner' -but made with text words. Depending on the website hosting the AdWords, Google shows a kind of ads or another, because visitors are interested in different areas. So, by targeting advertising, Google tries to increase the punch. By defect, if no results are found, charity organizations ads are shown (Red Cross, Cancer Societies, ...).