What is a skill?
How prompts, skills and agent packages differ
Prompt
A specific instruction you give a model, usually for one task or one step of a task. The product description should state the inputs it needs and the shape of the output.
Skill
A package of instructions and resources that teaches an agent tool one way of doing a job. SKILL.md is the main guide, and the package may also hold reference files or scripts.
Agent package
It brings together a role, its limits, the working steps and the tools it needs. Buying or downloading a package does not by itself run anything or connect any tools.