Refers to advertising, particularly on-line advertising, that is placed over the content. Also called shoshkeles. To read content, the user must either move the ad out of the way, close the ad, or wait for it to move away on its own. This is arguably even more bothersome than pop-up ads or popunder ads, since the user must deal with ads that scurry around the current window. Creative approaches to this have included animated blimps, the once-used Geocities collapsable ad-box, and all sorts of bouncing cartoon characters.
These ads are always custom-created by high-priced web consultants, and sometimes use pretty slick DHTML tricks. Because they are much harder for newbies to close, block, or ignore, they tend to be more "effective" than the simple banner ads of years past.