There are several types of Banners that are available...
The 480 x 60 banner is normally used at the top or bottom of the page and is prefferred by most..
The Small Button Banner is 120 x 60 and is usually place at the sides of the website content. When placed in the appropriate place can draw a lot of attention, especially when animated. The Square Banner 120x120 / 60x60 is convenient because it can fit almost anywhere. Mostly the Banners are placed around the site content but in special circumstances, can be placed inside the content as well.