Glossary · Platforms and measurement
Ad server
A system that stores ads, delivers them to pages, and counts impressions and clicks.
In detail
The ad server is the software that decides which ad is shown in each slot, delivers it to the page, and records what happens: how many times it was seen (impressions), how many clicks it got, and more. Well-known examples are Campaign Manager 360 and Google Ad Manager.
It is also where the banner is uploaded. That is why the package has to meet its rules: declared size, clickTag, maximum weight, and no external resources.
Example
You upload the .zip to Campaign Manager 360, assign it to a campaign, and the server delivers it to the sites where you bought space.