Skill vs Slash Command
Loaded automatically when relevant, versus triggered deliberately by name
Skill
A folder of instructions with a description the model reads at session start. When a task matches, the model loads the skill's full instructions on its own, without anyone needing to name it explicitly.
Slash Command
A saved prompt template triggered explicitly by typing /name. It runs only when a person calls it by name, never automatically based on the model judging it relevant.
Which one do you actually need
Use a Skill for knowledge the agent should recognize and reach for on its own, an API's conventions, a house style, a repeatable domain procedure. Use a Slash Command for an action a person wants to trigger deliberately, on demand, like a release checklist or a review pass that should never fire without someone explicitly asking for it.