Content Delivery Network. It is basically a network that is references by websites in order to access common resources, which are often cached by the clients' browsers. For example, many websites around the internet use jQuery. Google offers a CDN for jQuery so that websites can refer to that CDN each time they want the client to load it. However, since this reference is shared among many websites, chances are that the browser will already have this file cached from another website, so it does not have to download it again.