What does it mean to set the declaration of a function equal to 0? How can you assign an integer to a function?

I was browsing through the sources of a (prefer not to name) GUI Toolkit which wrapped up the Windows API when I found the following function definition in the window class:

 virtual LRESULT CALLBACK wndProc (HWND, UINT, WPARAM, LPARAM) = 0;

What is happening here? How can you assign a function to an integer? Or does it assign it to NULL? Do you need to do this if you want to use function pointers in the wndproc?