Understanding Template meta programming concepts & terminology

I have tried all source available on net and books not able to understand the concepts and syntax for the template.

template <int N>
struct Factorial 
{
    enum { value = N * Factorial<N - 1>::value };
};

template <>
struct Factorial<0> 
{
    enum { value = 1 };
};

// Factorial<4>::value == 24
// Factorial<0>::value == 1
void foo()
{
    int x = Factorial<4>::value; // == 24
    int y = Factorial<0>::value; // == 1
}

what is the "value" which associate with "Factorial<4>::value" why we need enum here. what is the difference between struct Factorial<0> and struct Factorial, why if i am declaring like struct Factorial<4>, why its giving error