A size-and-format specification for an ad. The Interactive Advertising Bureau, a trade association promoting digital ad standards and best practices, and guidelines like the Advertising Creative Guidelines. In essence, ad units are the individual ad spaces on a website or mobile app where ads are displayed. They come in different sizes and formats, such as banners, interstitials, native ads, video and expandable ads. There is great overlap between the concept of ad creatives and adunits.

Banners
The most common ad units, and they come in a variety of sizes, such as leaderboard (728×90), skyscraper (160×600), and square (250×250). Banners are typically placed at the top or bottom of a webpage.

Interstitials
Ads that appear between content pages, such as between levels of a mobile game. They can be full-page or smaller, and can be timed or user-initiated.

Native ads
Ads that blend in with the surrounding content, and can take many forms, such as in-feed or recommended content.

Video ads
Ads that are displayed in video format and can be pre-roll, mid-roll or post-roll.

Expandable Ads
Ads that can expand when clicked on, and can take up more space than the initial ad unit.

Each ad unit has its own distinct characteristics and can be used for different objectives, such as driving brand awareness, website traffic, or app downloads. Advertisers and publishers can choose the ad unit that best suits their needs, and ad exchanges facilitate the buying and selling of these ad units in real-time through a process known as real-time bidding in oRTB.