VS, using and "Add Reference" differences

The reference indicates that you can use the assembly, and a using is used to make it easier for you to use the assembly without having to write the full name.

such as:
after referencing System.Windows.Form, you can use the MessageBox.Show () method;
but before you use the System.Windows.Form, you have to do this:
System.Windows.Form.MessageBox.Show ()
when you add a using System.Windows.Form to your code, you can easily use it directly:
MessageBox.Show ()
The use of the "class. Method"
Of course, I use static methods here, other methods need to instantiate class first.
whatever you want to use must come from one of the assemblies you reference ,A reference is a reference to another assembly by an assembly;

A refers to B, which indicates that a must have a B assembly in order to function properly, possibly for the following reasons: Resource, code, and so on; If you do not find this Assembly B, assembly A will fail to start.
Using, in a unit of code, represents a reference to a namespace that allows your code to use the name in that namespace without the need for a full name.

Speak more, in fact this truth is very simple is not.

Vs, using and add reference differences