Should I use 'using namespace std'

Question

Should I use using namespace std; ? I saw some posts saying that using namespaces is a bad practice. So I just want to clarify this.

#include<iostream>

using namespace std;

int main()
{
    cout<<"Hello World"<<endl;

    return 0;
}

Or should I keep going like this

#include<iostream>

int main()
{
    std::cout<<"Hello World"<<std::endl;

    return 0;
}

Also please elaborate the reason.