Advertising networks is a company that links websites that want to host advertisements with advertisers. The main goal of advertising networks is to put a relevant ad on a relevant website. For example, if you are on a computer blog, there will be ads showing up about laptop sales instead of about car sales.
Advertising networks are a great way to increase publicity in your current target audience. When done correctly, advertising networks combine the best aspects of a website and an ad to make the user really want your product.