.NET Core is an open source, cross-platform development platform maintained by Microsoft that allows developers to build applications that run on Windows, Linux, and MacOS. It is designed to be fast, flexible and modern. Key characteristics include being open source, cross-platform support for major operating systems, flexible deployment options including framework-dependent and self-contained, compatibility with .NET Framework and command-line tools. The .NET Core platform consists of a runtime, fundamental libraries, SDK and compiler tools, and an app host.