Amadeus has a well-defined graphic chart covering both static and animated web banners. The trend is now away from animation to avoid the 'Christmas tree' effect of too many things trying to attract viewers' attention and preventing them from seeing the information that they need.
Static banners need to give a clear, direct message. Visuals are common to other communication media.
Animated banners can tell a more complex message, but beware of having several of them on the same web page!