Functions, what are they, how to use them and how to make them?

functions-1.cpp

#include <iostream>
#include <string>
using namespace std;

string GetName()
{
	cout << "What's your name? ";
	string name;
	cin >> name;

	return name;
}

void SayHello(string name)
{
	cout << "Hello " << name << endl;
}

int GetAge()
{
	int age;
	
	do
	{
		cout << "How old are you? ";
		cin >> age;
	} while (age <= 0); // make sure the users gives a positive number

	return age;
}

int Add(int num1, int num2)
{
	return num1 + num2;
}

void main()
{
	SayHello(GetName());
	
	int age = GetAge();
	cout << "So, you are " << age << " years old.\n";
	cout << "In 3 years, you'll be " << Add(age, 3) << " old.\n";
	
	system("pause");
}

functions-2.cpp

#include <iostream>
#include <string>
using namespace std;

// Prototypes
int Add(int, int);
int GetAge();
string GetName();
void SayHello(string);


void main()
{
	SayHello(GetName());

	int age = GetAge();
	cout << "So, you are " << age << " years old.\n";
	cout << "In 3 years, you'll be " << Add(age, 3) << " old.\n";

	system("pause");
}

/*
	Adds two numbers together
	@num1 the first number
	@num2 the second number
*/
int Add(int num1, int num2)
{
	return num1 + num2;
}

// Asks the user for his/her age
int GetAge()
{
	int age;

	do
	{
		cout << "How old are you? ";
		cin >> age;
	} while (age <= 0); // make sure the users gives a positive number

	return age;
}

// Gets the name of the user
string GetName()
{
	cout << "What's your name? ";
	string name;
	cin >> name;

	return name;
}

// Greets the user
void SayHello(string name)
{
	cout << "Hello " << name << endl;
}