Either say
Code:
#include <iostream.h>

int main()
{
     cout << "Hello World" << endl;
     return 0;
}
or

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

Code:
#include <iostream>
int main()
{
     std::cout << "Hello World" << std::endl;
     return 0;
}
Note: One of the members provided a link, please visit that. It will explain a lot of details about namespaces