Many people don’t like flashy advertisement banners on Web pages. But ads are a necessary thing for some pages to keep them free and help the owners pay their hosting fees. That might have been one of the reasons the bad guys thought of when using malicious banner ads as an attack vector. I’m not talking about the annoying banners that will overlay half of your screen so that you have to click them away manually. I’m talking about malicious ads, sometimes referred to as "malvertisement" or "badvertisement," which contain a malicious script or a hidden redirector. Most of the time it’s a flash object that contains an obfuscated action script which redirects the user to a malicious site after performing some user client checks.

Source: http://www.spywareremovalnews.com/news/article-1523.html