Ad rotation is the practice of showing multiple advertisements in a single location on a web page.
Service-based ad rotation is similar to client-side ad rotation in that it typically involves placing a small amount of JavaScript in the web page, but the JavaScript calls against a third-party service that provides its own management interface for specifying ad delivery options.
Google's AdSense service is an example of service-based ad rotation.
[1] These ads are shown in a variety of web sizes with the most common being a medium rectangle (300px x 250px) and a leaderboard (728px x 90px).
This strategy is against the TOU for Google’s Adsense ad rotation program.