Use of int, return, and void

I am new to C programming and I would like the following clarified

int main ()

{
printf("Hello World!");

return 0;

}

Why is the int used before main when the function does not return an integer but a string?
Why is return 0; used when function prints a string to the output?

 

You already had this thread once here and this is not your introduction and so I moved to C Programming and so make sure you create the thread once and possibly in the right forum.