What does Python's builtin __build_class__ do?

In Python 3.1, there is a new builtin function I don't know in the builtins module:

__build_class__(...)
    __build_class__(func, name, *bases, metaclass=None, **kwds) -> class

    Internal helper function used by the class statement.

What does this function do? Why must it be in builtins if it's internal? What is the difference to the type(name, bases, dict) function?