"A web banner or banner ad is a form of advertising on the World Wide Web delivered by an ad server. This form of online advertising entails embedding an advertisement into a web page. It is intended to attract traffic to a website by linking to the website of the advertiser. The advertisement is constructed from an image (GIF, Flash, often employing animation, sound, or video to maximize presence. Images are usually in a high-aspect ratio shape (i.e. either wide and short, or tall and narrow) hence the reference to banners. These images are usually placed on web pages that have interesting content, such as a newspaper article or an opinion piece.
The web banner is displayed when a web page that references the banner is loaded into a web browser. This event is known as an "impression". When the viewer clicks on the banner, the viewer is directed to the website advertised in the banner."
You can get an animated banner for only $99 and a static banner for $49.