What are CLR, CTS, and CLS?

CLR: The common language runtime consists of components that load the intermediate language code of a program into the runtime, compile the IL code into native code, execute and manage the code, enforce security and type safety, and provide thread support and other useful services.

CTS: Common Type System. This is the range of types that the .NET runtime understands, and therefore that .NET applications can use. However note that not all .NET languages will support all the types in the CTS. The CTS is a superset of the CLS.

CLS: Common Language Specification. This is a subset of the CTS, which all .NET languages are expected to support. The idea is that any program, which uses CLS-compliant types, can interoperate with any .NET program written in any language. Interop means allowing a c# class to inherit from a vb class.