The classic Online Advertising Form. First seen 1994 on http://www.wired.com.

Can have different forms, usualy expressed in pixels. The most common sizes:

Fullbanner 486×60 pixels
Halfbanner 234×60 pixels
Button 120×90 pixels
Leaderboard 728×90 pixels

Standard sizes are defined by the Internet Advertising Bureau (http://www.iab.com/pages.php?page=6).