http://blog.moertel.com/ does a decent job of explaining some practical aspects for de-recursivying, but I'll give you a basic explanation of that plus a more theoretical answer showing that any computatble function can be written using only while loops and a stack.

Practical: If we right all of are programs and tail-recursive (returning a recursive call or constant at the end of the prgram) programs they are easy to transform into simple while loops with no stack. The idea is instead of calling another function just restart the current one with the updated arguments.

EX:

    f(n,x)=f(n-1,x+1)
    f(0,x)=x

goes to

    while(n > 0){
      n--;
      x++
    }

Other techniques such as trampolining and continuation passing style also help translate recursive functions into iterative ones.

Theorectical: Think about Brainfuck or Turing machines, these computing frameworks can compute any computable function using only what amounts to while loops and infinite memory.

Given then that we can produce arbitrary control flow using only while loops, we can reproduce how recursive calls are computed in computers with stack frames (provided by our stack).

Here's a recursive directory traversal in Python:

  import os
  import os.path
  
  def list_files_recursive(path):
      filenames = os.listdir(path)
      for filename in filenames:
          fullname = os.path.join(path, filename)
          if os.path.isdir(fullname):
              yield from list_files_recursive(fullname) # recursively explore folders
          else:
              yield fullname

and here is the same program with an explicit stack

  import os
  import os.path
  
  def list_files_stack(path):
      folders = [path] # create stack of unexplored folders
      while folders: # repeat until stack is empty
          path = folders.pop() # pick a folder from stack
          filenames = os.listdir(path)
          for filename in filenames:
              fullname = os.path.join(path, filename)
              if os.path.isdir(fullname):
                  folders.append(fullname) # push folder to stack
              else:
                  yield fullname

Wow, somehow I expected something complex.

Thanks a lot!

Lets say you have two functions. Function A and function B. Function A does a bunch of stuff and function B is executed in the middle of function A. Well when function B returns the program must remember the context of function A in order to execute the remaining steps. It does this by storing the entire context of function A on a stack. This is known as the 'call stack.' Essentially, all recursive functions can and usually are implemented using a stack.