Skill vs Plugin
One teachable capability, versus a whole bundle of them installed together
Skill
A single folder covering one capability: a description, instructions, maybe scripts or references. The smallest unit of teachable, on-demand knowledge in this glossary's building blocks.
Plugin
A packaged bundle that can contain several skills, hooks, and commands at once, distributed and versioned together as one install. The wrapper, not the capability itself.
Which one do you actually need
A Skill is the unit, a Plugin is the packaging. Build a single Skill when one capability stands alone and gets reused on its own. Build or install a Plugin when several related capabilities, a house style's skills, hooks, and commands together, should travel and version as one set instead of being assembled by hand every time.