What does a C++ program require to run?

This question has been bothering me for a while now. Let's consider the two following programs:

#incude <iostream>
int main()
{
   std::cout << "Hello, World!";
}

and

int main()
{
   int x = 5;
   int y = x*x;
}

1. Windows: The first example, naturally, requires some system .dll's for the console. I understand that. What about the second? Does it need anything to run? Some runtime libraries? By the way, what do runtime libraries actually do?
2. Linux: No idea, can you enlighten me?

I know it depends on the compiler and OS, but I need either a general answer or particular examples. TIA.