What is it ?

Recursion in computer science is a method where the solution to a problem depends on solutions to smaller instances of the same problem (as opposed to iteration).

b4           = 4 * b3
             = 4 * (3 * b2)
             = 4 * (3 * (2 * b1))
             = 4 * (3 * (2 * (1 * b0)))
             = 4 * (3 * (2 * (1 * 1)))
             = 4 * (3 * (2 * 1))
             = 4 * (3 * 2)
             = 4 * 6
             = 24

Why recursion?

Where it's useful?

Let's see what notion do you have?

Function 1

void recursiveFunction(int num) {
   printf("%d\n", num);
   if (num < 4)
      recursiveFunction(num + 1);
}

Function 2:

void recursiveFunction(int num) {
   if (num < 4)
      recursiveFunction(num + 1);
   printf("%d\n", num);
}