Ubuntu Server is a part of the larger set of Ubuntu products and operating system developed by Canonical Ltd. Ubuntu server is a specific addition that differs a little bit from Ubuntu desktop, in order to facilitate installation on servers.
Techopedia explains Ubuntu Server
Ubuntu tools, which are open-source software, are alternatives to various types of licensed products. The word "Ubuntu" comes from a South African term for togetherness and sharing.
Some of the differences with Ubuntu Server include the replacement of a graphical user interface with a character-based interface for installation, as well as other changes to the interface. These can be customized by the user. The system is compatible with many different platforms like Microsoft Hyper-V and VMware ESX server.
Ubuntu Server can be an easy and convenient way to set up a simple home network. One other feature of Ubuntu Server is the ability to assign "super user" tasks to make it easier to administer the network, where using the original edition can be more challenging or labor intensive.