The changes thet made for ASP.NET 5 were based on customer requests and feedback. These changes simplify development, hosting, and maintenance, and are targeted to modern web apps. ASP.NET 5 gives you greater flexibility by being able to run on three runtimes:Full .NET CLRThe full .NET CLR is the default runtime for projects in Visual Studio. It provides the entire API set and is your best option for backwards compatibility.

Core CLR (cloud-optimized runtime)

The Core CLR is a lean and completely modular runtime for ASP.NET 5 projects. This CLR has been re-designed into components so you have the flexibility to include only those features that you need in your app. You add the components as NuGet packages. When you are finished, your app is dependent only on required features. By re-factoring the runtime into separate components, we can deliver improvements to the components more quickly because each component is updated on its own schedule. The Core CLR is about 11 megabytes instead of around 200 megabytes for the full .NET CLR. The Core CLR can be deployed with your app and different versions of the Core CLR can run side-by-side (both of these advantages are described in greater detail below).

Cross-Platform CLR

We will release a cross-platform runtime for Linux and Mac OS X. When released, this runtime will enable you to develop and run .NET apps on Mac and Linux devices. We will work closely with the Mono community on this effort. Until its release, you can use the Mono CLR for cross-platform development.