Banner ads can be of five main types

Text ads
Text ads are the "simple ones" that are available. The text must be clear that the link is an advertising link.

Image ads
These ads usually combine images  (gif, jpeg format etc) with a little bit of text. Images are more visually engaging than plain text however care must be taken that file size should remain small.

Flash ads
Macromedia Flash animated banners area a popular one. Care must be taken by designers however, that users may not have support for Flash; hence an alternative message should be displayed with a possibility to follow the advertising link.

Javascript/Pop-up ads
These are banner ads that are generated by JavaScript code. This means that the user must have the script engine activated. JavaScript is also used for pop-up the banners. Pop-up windows are ads launched into a new, smaller browser window when a page is loading. Pop-up windows can easily be targeted to launch over a specific page, a feature that not all interstitial technologies provide. Pop-up windows are most commonly run by attaching JavaScript launch code to a banner ad. This allows the pop-ups to be frequency-capped or targeted using any ad management system's targeting capability. Because pop-up windows are simply smaller browser windows, they can contain any standard Web display format, including HTML, GIF, JPG, and Macromedia Flash.

Skyscreaper ads
Also called Redwoods or Towers, these ads are usually found off to the sides of web pages. They provide a large advertising space while utilizing normally unused web space.
No matter what sort the ad is, if it has to be effective it must load fast, be attractive and make the user act. If it doesn’t do that; it’s being flashy is of no use.