maven vaadin plugin goals

Hi
I am new in Vaadin. I don't understand what exactly do goals: update-widgetset and compile.
When (in what cases exactly) should I run compile and when update-widgetset? Should they always be run together?

AB

 updates the Widgetset file itself, compile compiles widgetset based on the widgetset file.
so if are adding/removing/editing addons which has client side components, then you should run both (first update-widgetst then compile).
If you don't have addons that has client-side components, then run compile itself should be enough.

Thank you for reply.
You mentioned about addons but what with my own custom widgets?
Can you explain me what is "a widget set"? Is somehow "the widget set" connected with a module descriptor (.gwt.xml) file?