What is CLS (Common Language Specification) ?

It is a subset of CTS. If we want the code which we write in a language to be used by programs in other languages then it should adhere to the Common Language Specification (CLS). The CLS describes a set of features that different language have in common. The CLS defines the minimum standards that .NET language compilers must conform to, and ensures that any source code compiled by a .NET compiler can interoperate with the .NET Framework.
The CLS is a specification that defines the rules to support language integration. This is done in such a way, that programs written in any language (.NET compliant) can interoperate with one another. This also can take full advantage of inheritance, polymorphism, exceptions, and other features.