Ad server
The system that receives an ad request, decides which ad should run, serves it and records what happened.
An ad server sits between the placement that wants an ad and the sources that can supply one. For each request it applies the owner’s rules, asks the chosen demand, picks a winner and returns the creative or a pointer to it. It then counts the impression and any events that follow.
Publisher-side ad servers answer to the owner of the inventory. Advertiser-side ad servers host creatives and count delivery for the buyer. The two often talk to each other on the same impression.