An ad server is a technology platform used by advertisers, agencies, and publishers to manage and serve ads, track performance, and optimize campaigns. It acts as a central hub for running ad campaigns, from setting up tags to measuring results.

3rd Party Ad Servers: Primarily used by advertisers to track campaign metrics and verify performance. Examples include Google Campaign Manager, Sizmek, and Flashtalking.

1st Party Ad Servers: Used by publishers to control ad slots on their own properties, displaying ads sold directly to advertisers.