What is a Crawler?
Crawlers are banners that are displayed in a floating layer that adheres itself to bottom of the browser window. You might have also heard them called catfish banners before. Whatever you like to call them, the major draw is that as a visitor scrolls down your page the crawler stays put. This simple technique guarantees 100% viewable impressions because the crawler is always above the fold!
– See more at: http://www.advertserve.com/blog/2013/03/crawler-banners/#sthash.d5CZB2dU.dpuf