.NET Core is a set of libraries and a new common language run time. .NET Core is available as an open source software and it targets Microsoft,Linux and Mac. Microsoft has completely rewritten the existing .NET framework to create a light weight modular version of the .NET framework to support cross platform application development. .NET Core is modular because it is released as smaller assembly packages through NuGet. It's a subset of the full .NET framework as most of the unrelated vertical's of the previous .NET Framework versions are removed.
.NET Core is portable because you can package and deploy the CoreCLR along with the application and that eliminates your applications dependency on the installed version of the .NET framework.Using .NET Core,We can build different kind of application like web applications, console applications and native mobile applications. The .NET Execution Environment (DNX) provides a cross-platform run time host using which we can build applications that can run on different platforms.Applications running on DNX can target .NET Core or .NET Framework.