Windows Server 2016 is the successor to Windows Server 2012 R2. Built upon the same core code as Windows 10, Windows Server 2016 brings enhancements in security, servicing, and connectivity. A particular focus on this release was hybrid-cloud scenarios, and has close ties to Azure and other Microsoft cloud initiatives. This does not detract from the many improvements that are available for on-premises-only deployments
Windows Server 2016 comes in Datacenter, Standard, and Essentials editions, and for servicing, has adopted windows 10's cumulative model. The new nano-server install is designed to be remotely managed and is designed to be kept current through continuous feature updates. The full GUI install operates similarly to windows 10's "Long Term Servicing Branch" (LTSB) model with cumulative security updates.
Windows Server 2016 has also shifted from a per-processor-and-CAL licensing model to a per-core-and CAL licensing model. This brings Windows Server's licensing more in line with Microsoft's other products and makes hybrid-cloud license planning easier as well.