This allows you to opt some routes of your application into the new behavior while keeping other routes in the pages directory for previous behavior. The app directory works alongside the pages directory to allow for incremental adoption. The new App Router works in a new directory named app. URL Path: Part of the URL that comes after the domain (composed of segments).URL Segment: Part of the URL path delimited by slashes.Leaf: Nodes in a subtree that have no children, such as the last segment in a URL path.Root: The first node in a tree or subtree, such as a root layout.Subtree: Part of a tree, starting at a new root (first) and ending at the leaves (last).For example, a component tree with parent and children components, a folder structure, etc. Tree: A convention for visualizing a hierarchical structure.You will see these terms being used throughout the documentation. This page will introduce you to the fundamental concepts of the new routing model. Next.js 13 introduced the new App Router built on top of React Server Components with support for layouts, nested routing, loading states, error handling, and more.

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