We had an email from a prospect today that comes up with people that are new to PPC. Here is the question: What is the content network? I still don't really understand what that is?
Answer:
The content network are sites that have opted to display Google text and banner advertising. You have probably seen this type of advertising on newspaper sites or other publishers. Rather than showing the ads based on the search query (since there isn't a search), the ads are displayed based on the content on the screen. One of the best sources for the content network is Gmail. In Gmail, the email user sees ads related to the topics in the specific email. So if you are writing about your upcoming trip to San Francisco... a tour company, restaurant or hotel may want to have an ad next to that email.
The other benefit of the content network is that it allows the advertising to more effectively push their message to potential customers instead of the customer having to actively search. In the example above, the hotel company might be able to entice the user to check rates and book with them even if that person would have normally gone directly to Marriott or Starwood's website.
The one downside is that the purchase intent is usually lower since the prospect wasn't actively searching for the specific category of business. This is usually off-set by a much lower average cost per click.
I've also included a sample screenshot take from Gmail: 
On another note, I'm not sure why Business.com or BookSurge.com are coming up in the email. They aren't very relevant to the content -- although I didn't give Google much to work with.