Introduction

A function calling itself is called Recursion.
Example :

void hello() {
    System.out.println("Hello");
    hello(); // This is recursive call
}

When to use it ?

Recursion is usually applied to problems which can be broken down into smaller subproblems.
Example : Find factorial of a number

Factorial of a positive number $n$ is defined as :

Base case

Base case is the condition that stops the recursion. Usually it is the condition for which result is already known. $n = 0$ is the base case for the factorial problem where we already know the answer as $1$

Recursive case

The recursive case in recursion is the part of the function that reduces the problem into smaller subproblems and makes a recursive call to solve these subproblems. It's the part of the function where the function calls itself, typically with different parameters, until it eventually reaches the base case.

int factorial(int x) {
    if (x == 0) return 1; // Base condition
    return x * factorial(x - 1); // Recursive call
}