Since July, 2018, Google Ad Manager combines former DoubleClick for Publishers (DFP) and DoubleClick Ad Exchange (DCAE) products. It is a comprehensive hosted ad serving platform that streamlines your ad management, whether you deliver ads to websites, mobile webpages, mobile apps, games, or a combination. Yes, it can be used as an ad server but it also provides a variety of features for managing the sales process of online ads using a publisher's dedicated sales team.
It is a powerfull tool, but not the only tool you can use more effectively with a bit of Improvado assistance.