.NET Core is the next generation of the .NET framework developed by Microsoft, designed as the first cross-platform application framework for Windows, macOS, and Linux, with future support planned for FreeBSD and Alpine. It addresses needs for cross-platform compatibility, microservices, Docker containers, high-performance systems, and side-by-side .NET versioning for applications. The document also includes a demo on architecture, compilation, porting, and debugging .NET Core libraries.