Using Declarations versus Using Directives

A using-declaration is different than a using-directive.

A using-declaration brings one name into a scope. This name can be used as if it was declared locally.
For example,
    using std::vector;
Note that the keyword 'namespace' is not used.
A using-directive brings all the names of the namespace into a scope.
For example,
    using namespace std;
You can also have a namespace alias, which can be used make a namespace name shorter or clearer.
For example,

namespace WHAT_YOU_SEE_IS_WHAT_YOU_GET {
    void widget();
}
namespace WYSIWYG = WHAT_YOU_SEE_IS_WHAT_YOU_GET;

Reference: The C++ Programming Language by Bjarne Stroustrup. Addison-Wesley, 2000, Section: 8.2.2-3