A plasmoid is the combination of files that go into creating a plasma plugin. This includes:
* metadata (e.g. .desktop files) * svg images * configuration definitions (KConfigXT) * code (either something Kross groks or a compiled c++ library) * ...
An applet is the code part of a plasmoid, technically because it provides a Plasma::Applet object. To users, this will be an implementation detail they probably never see.
The Corona, a QGraphicsScene subclass, is the Plasma canvas. It contains all of the Containments that exist for the application, providing the "model" for the Views to use.
It handles initiating loading and saving of Containments and Applets and other such canvas-global tasks.
What contains the plasma. An applet that can contain other applets - for example, the desktop and panel containments.
Powers the gears that connect to the applet