Banners can be static graphics, GIF animations or small Flash movies.  A banner can be the heading for a page such as on this page or can be advertising on the side or middle of the page.    They do not have to be a certain size unless the use is for a rotating banner service or placed on a site as advertising where size is a prerequisite.

Use banners in one form or another to add more information to your page and attract the notice of your visitors.