Arc is a library of React components and blocks for product interfaces: inputs, menus, tables, sheets, pricing, auth, settings and dashboards, each with motion that already feels right. Install any piece with the shadcn CLI and own the source. Every item ships with Markdown docs and there's an llms.txt, so AI coding tools install and use Arc correctly. The core is free and open source; Pro adds motion showpieces, larger blocks and complete templates.