Skills are one of those Claude features that get mentioned constantly and explained rarely. The short version: a Skill is a packaged set of instructions - sometimes with extra files and scripts attached - that Claude uses automatically whenever it's relevant, so you don't have to re-explain the same workflow every single time.
The simplest way to picture it
Think of a Skill as an onboarding document you'd hand a new team member for one specific type of task - "here's how we format quarterly reports," "here's our house style for client emails." You write it once. Claude reads it automatically whenever a request matches what it covers, then follows those instructions without you having to paste them into the chat again.
Two kinds of Skills
Pre-built Skills ship with Claude already. If you ask Claude to create or edit a PowerPoint, Excel, Word, or PDF file, it automatically uses a matching Skill behind the scenes to do that well - no setup required.
Custom Skills are ones you (or your team) build. You package your own instructions - your company's specific report format, your team's code review checklist, your particular way of summarizing meeting notes - so Claude applies them automatically the next time you ask for that kind of work, instead of you re-explaining it from scratch in every new chat.
How this is different from a Project
A Project holds context and files for one ongoing piece of work - a client, a campaign, a codebase. A Skill is a reusable instruction set for one type of task, and it applies automatically across many different chats and Projects, not just one. You could have a Skill for "how we write release notes" that fires the same way whether you're working inside three completely different Projects.
How this is different from MCP
MCP (covered in the connecting tools guide) gives Claude a live connection to an outside app - your calendar, Slack, your files. A Skill doesn't connect to anything; it's know-how, not access. The two combine well: MCP gets Claude the data, a Skill tells it exactly how you want that data handled once it's there.
Where you can actually use them
Skills work on claude.ai, inside Claude Code, and through the Claude API. On claude.ai, custom Skills are uploaded per person under Settings > Features on Pro, Max, Team, and Enterprise plans - they aren't automatically shared across your whole team the way a Project can be, so each person who wants a given Skill currently needs to add it themselves.
One thing worth taking seriously
A Skill can direct Claude to run code and take actions, which means a Skill from an untrustworthy source is a real risk, not a theoretical one. Anthropic's own guidance is blunt about this: only use Skills you built yourself or got directly from Anthropic, and audit anything else carefully before trusting it with real work.