What is the difference between types String and string?

Does anyone know the difference between String and string in TypeScript? Am I correct in assuming that they ought to be the same?

var a: String = "test";
var b: string = "another test";
a = b;
b = a; // this gives a compiler error!

Current version of the compiler says:

Type 'String' is not assignable to type 'string'.
  'string' is a primitive, but 'String' is a wrapper object.
     Prefer using 'string' when possible.

Is that a bug?