An ad tag is a piece of code that is inserted into an ad slot on a website or mobile app. When the web page or app loads, the ad tag sends a request to the ad server to retrieve the appropriate ad. The ad server then uses the information provided by the ad tag to determine which ad to serve to the user.

Ad tags come in a variety of formats, including HTML or IMG, JavaScript, iframe, and SafeFrame. These different types of ad tags are used for different purposes, such as displaying ads on web pages or mobile apps, or serving video ads.

Ad tags are an essential part of the online advertising ecosystem. They allow advertisers to target specific audiences and to track the performance of their ads, and they allow publishers to monetize their ad inventory and maximize their revenue from ad sales. Ad tags are typically implemented using JavaScript or HTML, and they are a key component of the ad serving process.

The term "tag" can also refer to non-ad tags or analytics tags, such as those used for web analytics. These tags are used to collect data about website visitors and to track their behavior on the site.