[Iron_Cross]

It saves typing...Take the hello world application for example:

With using namespace std;

#include <iostream>
using namespace std;
int main()
{
    cout << "Hello World";
    cout << endl;
    return 0;
}

Without using namespace std;

#include <iostream>
int main()
{
    std::cout << "Hello World!";
    std::cout << endl;
    return 0;
}

In a small application like that typeing std:: doesn't make too much difference, but in a large application typing all the namespaces can be a pain in the butt! So when you include a using statement, you no longer have to include the fully quallified name.