ScriptSpot allows me to categorize my SketchUp add-ins under 2 categories: Plugins and Scripts. The line between a script and a plugin is a little grey, I suppose - so here's what they mean for my content:
A Script is a single-file add-in for SketchUp.
A Plugin is a multi-file add-in for SketchUp, downloaded as a .zip file.
The category of script or plugin says nothing about the functionality of the add-in.
Simple enough?