classes i need help!!

i am learning c++ but i have a problem with classes;

Code:

1:   // Demonstrates declaration of a class and
2:   // definition of class methods,3:
4:   #include <iostream.h>      // for cout5:
6:   class Cat                   // begin declaration of the class7:   {
8:     public:                   // begin public section
9:       int GetAge();           // accessor function
10:      void SetAge (int age);  // accessor function
11:      void Meow();            // general function
12:    private:                  // begin private section
13:      int itsAge;             // member variable
14:  };
15:
16:  // GetAge, Public accessor function
17:  // returns value of itsAge member
18:  int Cat::GetAge()
19:  {
20:     return itsAge;
21:  }
22:
23:  // definition of SetAge, public
24:  // accessor function
25:  // returns sets itsAge member
26:  void Cat::SetAge(int age)
27:  {
28:     // set member variable its age to
29:     // value passed in by parameter age
30:     itsAge = age;
31:  }
32:
33:  // definition of Meow method
34:  // returns: void
35:  // parameters: None
36:  // action: Prints "meow" to screen
37:  void Cat::Meow()
38:  {
39:     cout << "Meow.\n";
40:  }
41:
42:  // create a cat, set its age, have it
43:  // meow, tell us its age, then meow again.44:  int main()45:  {
46:     Cat Frisky;
47:     Frisky.SetAge(5);
48:     Frisky.Meow();
49:     cout << "Frisky is a cat who is " ;
50:     cout << Frisky.GetAge() << " years old.\n";
51:     Frisky.Meow();
52;      return 0;53: }

why do we use itsage in private and why dont we use it in public??

You could put age in public if you wanted. The reason to make it private is to prevent other parts of code from messing around with it. If it was public, any code could change it. Making it private means only the class it's in can manipulate it, and your get/set functions control how that manipulation is done.