When a function calls itself to produce a result, it is said to be recursive. Sometimes recursive functions are very useful in that they make it easier to write code. Some algorithms are very easy to write using the recursive paradigm, while others are not. There is no recursive function that cannot be rewritten in an iterative fashion, so it's usually up to the programmer to choose the best approach for the case at hand.
A recursive function usually has a set of base cases for which the return value doesn't depend on a subsequent call to the function itself and a set of recursive cases, for which the return value is calculated with one or more calls to the function itself.
As an example, we can consider the (hopefully familiar by now) factorial function N!. The base case is when N is either 0 or 1. The function returns 1 with no need for further calculation. On the other hand, in the general case, N! returns the product 1 * 2 * ... * (N-1) * N. If you think about it, N...