The Windows Research Kernel AKA WRK
Is a part of the source code of the actual windows NT Kernel. WRK is designed for academic uses and research, by no means it can be used for commercial purposes.
Properties
This is actually part of the windows kernel, which means you can find the source code of many kernel-level NT API functions, Structures & Data that is not defined in the standard API headers.
Uses
Since you can find here the full source codes of many APIs and structures which aren't defined in the public headers WRK can be used to study the kernel so you can actually understand what is going on 'behind the scenes'. You can find potential exploits or make your kernel modules harder to reverse by using the actual source of a structure/API instead of importing it from ntoskrnl.exe.
To Be Noted
This is not nearly the full source code of the kernel, lots of files are missing or removed. Also, this code is still somewhat protected (although leaked). Commercial use of this code is highly prohibited. Use only for personal and research purposes.
Disclaimer: I am not affiliated with microsoft or any of it's trademarks. This source code belongs to it's respective owner(s). Windows, Windows Kernel, Software Development Kits & Other Windows' components are registered trademarks of Microsoft Corporation in the United States and/or other countries.