What is an ad server?
This title might sound tricky for anyone that is already familiar with ad operations, but since there is pretty much confusion about it and since we’ve s been repeatedly asked, it’s time we wrote a 101 article 
Here’s a simple definition of an ad server:
An ad server is a web server, that stores advertisements used in online marketing and delivers them to digital support visitors (a website, mobile apps, a mobile site,…).
Thanks a user-interface, users are enable to manage their banners, e.g to put online the creative & set the delivery parameters, (eventually) target , ad serve, monitor and optimize their online campaigns.
Different types of banners ( text, banner, video, flash, HTML5,…) can be supported.
As an ad server consists only of the technology, there is a prerequisite you might need before starting using one: advertisers willing to advertise on your digital support & the related creatives , even if an ad server can help building advanced banners (Smart Adserver does it pretty well for instance) but not making them from scratch.
Why you would need an ad server?
If you’re a sell-side actor (e.g you have inventory to sell) : you are a Publisher or an Ad Network
You’re a publisher, i.e you create digital content that you’re willing to monetize, including with advertising. You can make revenues by directly advertising to direct advertisers and agencies interested by your audience or through a third-party actor.
If you sell inventory directly to some advertisers, then you’ll probably need an ad server to manage all your advertising campaigns.
If you don’t have any direct advertiser, either because you’re working with automatic advertising platform, such as google adsense, or you’re dealing with an ad network that would sell you ad inventory on your behalf to advertisers, you don’t have any interest in managing yourself your banners with an ad server.
If you’re a buy-side actor (e.g you have inventory to buy) : Advertiser or Media Agency
You’re an advertiser, willing to advertise you banners to sell-side actors. You have already built our media plan. Or you are an agency, ad serving banners on behalf of your advertisers.
You might want to be able to track by your own the clicks, impressions, conversions of the banners you deliver.
