Display advertising monetezation is divided between Advertisers and Publishers. Both can add extra revenue if used properly.
Advertisers: As a Display Advertiser, you would provide your own creative, meaning images and text to place within standard IAB (Interactive Advertising Bureau) ad unit sizes. These ads are then placed onto other web sites to give your site visibility, and the ability for users to click through to your site. Generally speaking, you would pay a flat CPM (Cost per 1000 impressions) for this traffic. These costs can range from $0.25 to above $1.00 CPM depending on your web site's general topic and the type of traffic that you are looking for. So Assuming that you are paying $0.25 CPM, you would pay $0.25 for every 1,000 times your ad appears on other web sites.
Publishers: As a Display Publisher, you would make additional revenue by implementing ads from Display Advertisers on your web site. Once your site has been approved by ad networks, you would be supplied codes for each IAB size and placement on the page. You would then be paid on a CPM basis. Please note that these payments are based on individual Ad Network's rules. The most typical rule would be that they would only pay for 1 visit from any particulat I.P. address within 24 hours. There are other paramters that would be discussed on a site by site basis.
The most used Ad Unit sizes, in Pixels per Inch, for any typical web site would be 160x600, 300x250 and 728x90. Here are examples of each Ad Unit Size that you can advertise with or that can be implemented onto your site: