Ad tags are used for many purposes in display advertising:
For example, here's an AppNexus ad tag that a publisher would use to auction an impression.
document.write('<iframe frameborder="0" width="160" height="600" marginheight="0" marginwidth="0" target="_blank" scrolling="no"
A publisher ensures that the ad is an iframe.
A Google AdSense tag
In the NOSCRIPT section, the HREF link is the landing page of the ad and the SRC is the ad itself.