Ad server: Normally operated by a third party, an ad server delivers and tracks ads on websites. Ad servers perform a useful role in building trust between advertisers and publishers since the statistics they supply are likely to be free of spin.
Ad server content is updated on a regular basis to keep advertisements fresh and dynamic. There are two primary types of ad servers, local and third party. Local ad servers generally serve a single publisher and distribute ads to websites specifically owned by that publisher. Third party servers take ads from a central server and distribute them among multiple publishers. The advantage of a local server is that is allows for more specialization and quicker changes for a single publisher. The advantage of a third party server is that it has a larger distribution network.
Advertisers can, of course, use both if their budget allows for it.