More About the Ads

Google sells a service called AdSense. If you have a website that meets their criteria you can register for AdSense, have Google display ads on your website and you get paid per click when visitors to your website click on the ads.

The way it works is that Google scans the text on the webpage for matches on certain keywords ("AdWords") and then it displays ads for people who have paid to have their ads displayed in response to those keywords. I haven't looked into this in great detail but Google seems to have a sort of bidding system that lets the advertiser specify the keywords and how much they want to pay per click per ad. The net effect is that some keywords are worth more than others so, depending on what keywords Google finds on your website, you can make anything from pennies per click to several dollars per click.

I suspect that the context in which the keywords are found is also important. For example, keywords positioned in titles, headings, near the beginning of the page, etc., are more likely to trigger ads than keywords found in other places.