Nano server is the small, really small, footprint install version of Windows Server that was introduced with Server 2016.
it has a limited number of roles available to to install – much like the original version of Server core.
indicates that Nano server is going to become even smaller (by more than 50%) and dedicated to delivering Containers. The infrastructure related roles will be removed. Nano Server will ONLY be available as a container base OS image
In addition, starting this Autumn, Nano server and Server Core will getting 2 feature updates per year.