In March this year, Microsoft announced Windows Server 2019, the next Long Term Servicing Channel release of Windows Server. This also coincides with the next Semi-Annual Channel update, which is called Windows Server, version 1809.
Today at Ignite, the firm announced that they will be generally available beginning in October. Of course, this is no surprise. We're also expecting version 1809 of Windows 10 client and version 1810 of the Xbox One experience to be available next month.
The difference between Windows Server 2019 and Windows Server, version 1809 is mainly how updates will be released. If you're on the Long-Term Servicing Channel, you're going to receive cumulative updates, similar to how Windows Server 2016 has (Windows Server 2016 coincided with version 1607). Those on the Semi-Annual Channel will get the next feature update, which will likely be version 1903.
Windows Server 2019 also adds features that Semi-Annual Channel users have had for a while, but Windows Server 2016 users haven't received. It's meant to bridge the gap between on-premises servers and Azure services for a hybrid environment. Windows Admin Center version 1809 is also out, and it's meant to be complementary to these new Server versions.