.NET is an operating platform, designed by Microsoft, for developing software.
.NET Standard is a set of application programming interfaces (API) implemented by the Base Class Library of a .NET implementation.
.NET has 4 primary implementations.
.NET Core is an open-source managed software framework for Windows, macOS and Linux.
.NET Framework is a cross-platform, open-source framework for developing software.
Mono is an open-source project to create an ECMA standard-compliant and .NET Framework-compatible set of tools.
Microsoft provides information about .NET.