For the most part, Google's ads do a reasonable job of matching ads to content. Put simply, Google looks at a web page, works out what the page is about by looking at the placement and recurrence of words and then delivers ads that are in context. However, while researching the theological issues of the Jerusalem Council in Acts 15 I got the following.

You can get a full-sized (280kb) version of the image by clicking.

Google has taken the main subject, Christianity, and I think the word "seminal" on the right side next to the book's cover-page and come up with a link to an ad for Christian Masturbation (under the main text on the page.

 Google ads