Microsoft’s server line began with Windows NT 3.1 Advanced Server in 1993. Windows 2000 Server introduced Active Directory, which is still the directory service at the centre of most corporate networks. The “Windows Server” name arrived with Windows Server 2003.
Modern releases come out on the Long-Term Servicing Channel about every three years. Each gets five years of mainstream support and five more of extended support, and paid Extended Security Updates can add years after that. Microsoft tried faster Semi-Annual Channel releases from 2017 to 2022 before dropping them. It now offers Azure Edition and Azure Arc hotpatching for organisations that want more frequent changes.