Cosmos (C# Open Source Managed Operating System) is an operating system development kit which uses Visual Studio as its development environment. Despite C# in the name any .NET based language can be used including VB.NET, Fortran, Delphi Prism, IronPython, F# and more. Cosmos itself and the kernel routines are primarily written in C#, and thus the Cosmos name.
Cosmos is not an operating system in the traditional sense, but instead it is an "Operating System Development Kit". Cosmos lets you create operating systems just as Visual Studio and C# normally let you create applications. Most users can write and boot their own operating system in just a few minutes, all using Visual Studio.
Cosmos is available in two distributions, the developer kit (dev kit), and the user kit. The dev kit is designed for users who want to work on developing Cosmos itself. The user kit is designed for those who are interested in building their own operating system and doing some Cosmos work. The dev kit might be thought of as the Cosmos SDK. Users wishing to work on developing Cosmos should read the developers guide, and those who wish to create an Operating System should download the UserKit.