What is namespace used for, in C++?

Namespace is used to prevent name conflicts.

For example:

namespace foo {
    class bar {
        //define it
    };
}

namespace baz {
    class bar {
        // define it
    };
}

You now have two classes name bar, that are completely different and separate thanks to the namespacing.

The "using namespace" you show is so that you don't have to specify the namespace to use classes within that namespace. ie std::string becomes string.

It's used for preventing name confilct, so you may have two classes with the same name in different namespaces.

Also it's used for categorizing your classes, if you have seen the .net framework, you will see that namespaces are used for categorizing the classes. For example, you may define a namespace for the employee classes, and a namespace for the tasks classes, and both namespaces are within a namespace for the company classes, since a namespace may contain sub namespaces.

The same namespace may exist in different files, so using it may be useful because it will make you able to directly use all the classes in the namespaces in every #included file.

That's what I remember for now.