An overlay ad is a banner or message that opens on top of the page content, often when a reader arrives or scrolls. The reader usually has to close it before reading on.
How it's sold
One publisher, Allmusic, lists it, and you book through their sales team. Overlays are also sold programmatically by some networks.
What it costs
Most publishers quote this on request, usually per 1,000 impressions or as a flat monthly fee.
What you supply
You supply an image or HTML5 file in the stated size, and a link, plus a clear close button if the publisher asks for it.
Best for
It suits short promotions where attention matters more than comfort.
Before you buy
Check the frequency cap, since readers who see the same overlay repeatedly tend to leave.