The start of Windows Server can be discussed, one can argue that it started with Windows NT 3.1 back in 1993 or you can argue it really started with Window Server 2000. If you start with Windows NT 3.1 then you can follow the line through NT 3.5, NT 3.51, NT 4 and on to Windows 2000. Alternatively it was really Windows Server 2003 that gained the stronger server branding.
More information on Windows Server can be found on Wikipedia at Windows Server which I have to admit is a great resource.
Sometimes it is handy to know the latest service pack for a given Operating System, so here is a short and handy list:
This is the model of Windows Server releases that we have had for the last 20 years or so. Approximately every 2-3 years we see a new major release, which has mainstream support for 5 years and an additional 5 years of extended support. The release is full featured with a user interface, although recent versions will install as server-core, without a UI. These releases are fully supported and administrators remain in control of patching.
The "semi annual" channel has a new version every 6 months and is supported for 18 months, it is designed for like Windows 10 where you get regular updates automatically. However bear in mind the administrator can still control patching and these are supported releases. However they do not have a user interface, so are in effect "server core" releases. If you are wondering where the number comes fro with Windows Server 1709, it is from "September 2017".
If you need information on the different editions of Windows Server, then for 2012 Download Window Server 2012 Products and Editions Comparison from Official Microsoft Download Center however this does not seem to cover R2. One document that does is the Windows Server 2012 R2 Licensing Datasheet, which is on the "Buy" tab of Windows Server 2012 R2 | Microsoft and provides a good explanation and comparison, although lacks some detail in the previous document.
There is information under Microsoft Windows that will also apply to the Windows Server range of products.
The Windows Server documentation at Windows Server | Microsoft Docs explains the different release channels and gives details or Windows Server, Server Core and Nano Server. Note that Windows Server Semi-Annual Channel overview | Microsoft Docs gives a good explanation of the new Semi-Annual Channel.