Difference between local and public variables

public variables once declared they can be used any where in the program i.e. even in many functions while local variables are local to a functional and cant be used beyond that function.

#include <iostream.h>
   void myFunction();           // prototype

   int x = 5, y = 7;            // global variables
   int main()
   {

       cout << "x from main: " << x << "\n";
       cout << "y from main: " << y << "\n\n";
       myFunction();
       cout << "Back from myFunction!\n\n";
       cout << "x from main: " << x << "\n";
       cout << "y from main: " << y << "\n";
       return 0;
  }

  void myFunction()
  {
       int y = 10;               //local variable

      cout << "x from myFunction: " << x << "\n";
       cout << "y from myFunction: " << y << "\n\n";
 }

Difference between pre-defined and user-defined function

The user-defined functions are defined by a user as per its own requirement while library functions or pre-defined come with compiler.

Example: In the above code, main() function is pre-defined and myFunction() function is user-defined.

A function prototype is a declaration of a function that omits the function body but does specify the function's return type, name, arity and argument types. While a function definition specifies what a function does, a function prototype can be thought of as specifying its interface.

#include <iostream>
using namespace std;
void printMessage(void);  // this is the prototype!

int main ()
{
   printMessage(); 
   return 0;
}
void printMessage (void)
{
   cout << "Hello world!";
}