Google Search is like the Yellow Pages; people use it to find solutions to a problem or discover information about a product or service.
On the other hand, the GDN is more like billboards or magazine ads. It show ads where visitors are reading articles or watching videos and not necessarily searching for a particular solution to a particular problem.
The GDN allows you to reach over 90% of internet users with four targeting options.
The first three options are great for a very broad approach to targeting your ideal audience but what you really need is a more controlled approach to your advertising. This is why Managed Placement is the best way to start using the GDN so you have full control of where your ads are placed.
This option allows you to enter the web address of the sites where you want to advertise. If those sites are available, Google will show your ads there. You can even target a specific section on a website.