How do two structures with same name and with different Template arguments work

Why does only first implementation work and not the other implementations specified below, Can someone explain the way this template structure work and why others do not.

Template structures that work

template <typename T, typename U>
struct is_same
{
    static const bool value = false;
};

template <typename T>
struct is_same<T, T>
{
   static const bool value = true;
};

Template structures that do not work

template <typename T, typename U>
struct is_same<T, U>
{
   static const bool value = false;
};

template <typename T>
struct is_same<T, T>
{
  static const bool value = true;
};

One more that does not work

template <typename T, typename U>
struct is_same<T,U>
{
   static const bool value = false;
};

template <typename T>
struct is_same
{
   static const bool value = true;
};

And the Main Function

template <class A, class B>
bool IsSameClass() {
  return is_same<A, B>::value;
}
int main()
{
  bool ret =    IsSameClass<P,C>();
}