The process in which a function calls itself directly or indirectly is called recursion and the corresponding function is called as recursive function and such function calls are called recursive calls. Using recursive algorithm, certain problems can be solved quite easily.

Generally, iterative solutions are more efficient than recursion since function call is always overhead. Any problem that can be solved recursively, can also be solved iteratively. However, some problems are best suited to be solved by the recursion, for example, tower of Hanoi, Fibonacci series, factorial finding, inorder/preorder/postorder Tree Traversals, DFS of Graph, etc.

Recursion always consists of two main parts

1. A terminating case that indicates when the recursion will finish 
2. A call to itself that must make progress towards the terminating case.

How recursion works?

void recurse()
{
    ... .. ...
    recurse();
    ... .. ...
}

int main()
{
    ... .. ...
    recurse();
    ... .. ...
}

Example : Calculates the factorial of a given number using a recursive function

#include <stdio.h>

unsigned long long int factorial(int i) {

   if(i <= 1) {
      return 1;
   }
   return i * factorial(i - 1);
}

int  main() {
   int i = 12;
   printf("Factorial of %d is %d\n", i, factorial(i));
   return 0;
}

Output

Factorial of 12 is 479001600

What is the difference between direct and indirect recursion?

A function funRecursion is called direct recursive if it calls the same function funRecursion . A function funRecursion is called indirect recursive if it calls another function say funRecursionOuter  and funRecursionOuter  calls funRecursion directly or indirectly. 

An example of direct recursion

void directRecFun()
{
    // Some code....

    directRecFun();

    // Some code...
}

An example of indirect recursion

void indirectRecFun1()
{
    // Some code...

    indirectRecFun2();

    // Some code...
}
void indirectRecFun2()
{
    // Some code...

    indirectRecFun1();

    // Some code...
}