I am new to C++ Programming , What is the difference between these codes , and Which one I should use? [duplicate]

I got both the codes from Books from an Online PDF

First -

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

Second -

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