using namespace std

When we use names of variables, functions, etc., we use one unique name in one namespace.
 
We can use a variable name "num" in one namespace, and the same name in another namespace. And it can be used like the following:
 
N1::num
N2::num
 
Of course, they are different variables.
 
By typing "using namespace std" after typing "#include <iostream>", we can use names in the standard library directly like "cout" instead of typing something like "std::cout".