I cannot answer to the previous answer from thllbrg (which is roughly right ... Also Oren blog post is the most important source at the current time), but one important clarification: dotnet
does not have a runtime.
Libraries build with dotnet
can deploy the DNX, .Net Framework and the UWP platforms (if the dependencies of your libraries allow so). All have different CLRs and compilation models. DNX e.g. support compilation on the fly, while UWP compiles ahead of time into one file and does tree shaking (the killing of not used functions in your library).