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