Windows Server is a brand name for a group of server operating systems released by Microsoft since 2003. The first Windows server edition to be released under that brand was Windows Server 2003. However, the first server edition of Windows was Windows NT 3.1 Advanced Server, followed by Windows NT 3.5 Server, Windows NT 3.51 Server, Windows NT 4.0 Server, and Windows 2000 Server. Windows 2000 Server was the first server edition to include Active Directory, DNS Server, DHCP Server, Group Policy, as well as many other popular features used today.
Windows Server 2019 has three editions: Essentials, Standard, and Datacenter. As their names imply, they are designed for organizations of different sizes, and with different virtualization and datacenter requirements.