An ad tag is a code snippet that has to be inserted within the HTML code of a webpage where an ad is due to be displayed. An ad tag is created and placed for each ad placement on the page.
When the ad tag is uploaded by the user’s browser during a visit on the publisher website, the tag gives instruction to the browser to retrieve the ad from an ad server. It is called an ad request.
Most ad tags are based on Javascript and placed in iFrames. The ad tag is created on an ad server interface by an ad op or an ad trafficker. Below an ad tag used by AdSense publishers for inserting a skyscraper:
<script type="text/javascript">
google_ad_client = "ca-pub-xxxxxxxxxxxxxxxx";
google_ad_width = 160;
google_ad_height = 600;
</script>
<!-- 160X600 -->
<script type="text/javascript"
src="//pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
Source: Digital Marketing Glossary
