Of course, advertising is never effective until it is shown to the right group of customers. This is where “cookies” come in. Cookies are like unique signatures for computers or devices via which customers connect to the internet. Advertisers gather data about their prospective customers and their preferences. They track which are the sites their customer's visit, which ads the latter click and which they don’t. Based on the information they gather; the advertisers can do targeted advertising. Display marketing is an effective way to remarket to the target audience. However, you cannot just randomly create an ad and put it on a website. Careful thought has to go into planning and executing display advertising campaigns. To do that, we must first look into what ad networks are.