The banner is graphic image with definite sizes. He works like hyperlink toward website. The name comes from english Banner - flag, ensign. There are different file formats - JPG, GIF, PNG. Usually the size for one banner from type 300x250 is 40 KB, this is standard magnitude, which the file shouldn't exceed.
On one position can located several banners, replacing for period of time or at every page reload /banner rotation/.
The banner can easily updated, when you need.
The proportion between number of banner showing - impressions in relation to number of clicks is around 1%. The banner is most popular form for online advertisement.