Skills
Also: Claude Skills, Claude.ai skill, custom skill
Skills are packaged workflows that tell Claude exactly how to execute a specific type of task — the steps, the methodology, the output format. There are two kinds: Anthropic's built-in Skills (which handle Excel, Word, PowerPoint, and PDF creation automatically for paid users) and custom Skills you build for your own repeatable processes. A custom Skill might encode your brand review checklist, your QBR structure, or your quarterly analysis methodology. You create one by describing your workflow to Claude — it builds the Skill file for you. Once saved, Claude invokes it automatically whenever you do that type of work. The difference from Projects: Projects hold what Claude needs to know; Skills define how Claude should act. Note: Claude Code also has a feature called Skills (SKILL.md files) — a separate concept for developer workflows. See Claude Code Skills.
In practice
You ask Claude to turn a data summary into a real Excel file. Without the Excel skill, Claude describes what the spreadsheet would look like. With it, Claude actually produces a .xlsx file you can open directly. Skills extend what Claude can do beyond text — they're the capabilities that produce real files, not just descriptions of files.