what is the difference between the "iostream" library and "using namespace std;" in c++

I am new to C++. I can't get the difference between `iostream` library and `using namespace std;` because `iostream` library are two types named `istream` and `ostream`, which represent `input and output streams`, respectively. And `namespace std` is the `standard namespace`.` cout`, `cin` and a lot of other things are defined in it. (This means that one way to call them is by using `std::cout` and `std::cin`.)

So my question is, what is the need of defining both `iostream` and `using namespace std;` in a program?