What is the 'using' keyword in C++?

#include <iostream>
int main() {
  using std::string;
  using std::cout;
  string s = "Hello World";
  cout << s;
  return 0;
}

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

#include <iostream>
using namespace std;
class Base {
   public:
      void greet() {
         cout << "Hello from Base" << endl;;
      }
};
class Derived : Base {
   public:
      using Base::greet; 
      void greet(string s) {
         cout << "Hello from " << s << endl;
         // Instead of recursing, the greet() method
         // of the base class is called.
         greet();
      }
};
int main() {
   Derived D;
   D.greet("Derived");
   return 0;
}

#include <iostream>
using namespace std;
class Base {
   public:
      void sum(int a, int b) {
         cout << "Sum: " << a + b << endl;;
      }
};
class Derived : protected Base {
   public:
      using Base::sum;
};
int main() {
   Derived D;
   // Due to protected inheritance, all the public methods
   // of the base class become protected methods of the
   // derived class. If the using keyword word was not used,
   // calling sum like this would not have been possible.
   D.sum(10, 20);
   return 0;
}