A Skill in Claude Code is a specialized workflow that Claude can invoke automatically or you can trigger manually with a slash command. Unlike CLAUDE.md rules (which advise on every decision) or Hooks (which enforce deterministic actions), Skills are progressive disclosure — deep knowledge that loads only when relevant, keeping your base context lean while providing specialized expertise when needed. This guide covers how Skills work, when to use them, and how to build real Skills that serve developers, testers, and DevOps engineers.