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.
Where the activity happens
What contains the plasma
Powers the gears that connect to the applet