A written work instruction for a recurring task that Claude loads when the task fits.
A skill is a folder with a SKILL.md file that you upload as a ZIP file in Claude under “Customize” and “Skills”. Because Claude loads it only for matching tasks, it does not weigh on every chat, unlike project instructions. A skill pays off when you give the same correction for the third time. Anthropic ships its own skills, for example for Excel, Word, PowerPoint and PDF.
Example from Basics
In the course's fictional plant shop, the Customer Care team lead corrects every Friday that “Overdue” only lists items whose deadline has passed. The third time, she writes the rule down as a skill.
Basics: “Claude bedienen”. In the Werkbank course room, with sign-in.