What is Ad Server?

Also known as: Publisher ad server, Advertiser ad server, GAM

Quick definition

An ad server is the system that decides which ad to show for an impression and records delivery. Publisher ad servers manage direct deals and programmatic demand on owned inventory; advertiser ad servers host creatives, rotate them and count impressions and clicks across all media bought.

Key points

Publisher side: Holds line items with priorities, targeting and pacing, and competes them against exchange or header-bidding demand.
Advertiser side: Issues tags (including VAST tags) to every publisher so the brand has one consistent count of delivery.
Discrepancies: Publisher and advertiser ad servers count at slightly different moments, so a few percent of discrepancy is normal.

Benefits

Central control of delivery and reporting
Lets direct deals and programmatic compete

Trade-offs

Market concentration around a few vendors
Complex setup and line-item management

FAQ

Is an SSP an ad server?

Not exactly: an SSP sells inventory to programmatic buyers, while the publisher ad server decides between that demand and direct campaigns. Some vendors offer both.

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Official sources

IAB — Glossary

Last reviewed 2026-10-27. Explanations are written by AdTech Radar; consult the official specification for implementation.

Related terms

VAST Tag & Wrapper Chains

A VAST tag is the URL a video player calls to fetch an XML ad response. The response is either Inline (it holds the media files and tracking URLs) or a Wrapper that points to another ad server's VAST, so one impression can pass through several redirects before a playable creative arrives.

Header Bidding

Header bidding is a technique where a publisher offers each impression to multiple SSPs and exchanges simultaneously before calling its ad server, so demand competes on price at once instead of in a sequential waterfall.

Deductions & Reconciliation

Reconciliation is the month-end process where ad partners compare impression and revenue counts and agree what will be paid; deductions are amounts removed for discrepancies, invalid traffic or brand-safety violations, often months after delivery.