Namespace

Use entire std namespace

#include <iostream>

using namespace std;

int main()
{
    cout << "hello world";
    return 0;
}

Use only what you need

#include <iostream>

using std::cout;
using std::cin;
int main()
{
    int number;
    cout << "Enter a number: ";
    cin >> number;
    cout << "Your number is " << number << std::endl;
    return 0;
}