In orchard what responsibilit does a widget play and what role does a plugin satisfy

I see many CMS and frameworks use modules, plugins and widgets very loosely.

While I understand the goal is modular architecture, besides modules with the other two its unclear on what responsibilities do they full-fill

so specifically, from a) user perspective and b) developer perspective? what roles do the widget vs plugin play in the orchard framework, or are they the same thing?