TLDR: PowerShell Core v6 is a fork of PowerShell v5. All future development will happen on Core (BUT a lot of features of ver.5 didn’t find their way to Core ver.6).
PowerShell AKA PowerShell Desktop AKA PowerShell v1 to v5: It is and will continue to be a very stable and reliable platform that is not going to be significantly modified or gain new features. Microsoft is to only provide bug fixes and security updates to PowerShell 5.
PowerShell Core AKA PowerShell v6,7,…: It’s multiplatform(Linux, Mac, Windows) and it’s the product that will get all the new features going forward. So this will be the choice for those who want to have the latest and greatest capabilities. The first version of PowerShell Core (ver6) was a fork of PowerShell 5 that didn’t have all the features of PowerShell 5. Even in the Windows version of PowerShell Core 6 a lot of cmdlets that were found in v5 are missing. And not only that but there are also entire modules missing. On the other hand a lot of new features & cmdlets & modules first appeared in v6 and v7 and all new features are going to be developed for PowerShell Core only.
(This answer is mainly based on http://techgenix.com/powershell-core/)