What is App Router?
Next.js 13+ routing system built on React Server Components, with file-based routes under `app/`, nested layouts, and streaming.
The App Router is Next.js's routing primitive starting in version 13, replacing the older Pages Router for new development. Routes are defined by folder structure under `app/` — each folder represents a URL segment, and `page.tsx` files are the leaf pages. Layouts (`layout.tsx`) wrap nested routes and persist across navigation, eliminating full-page rerenders. Special files include `loading.tsx` (Suspense boundary), `error.tsx` (error boundary), `not-found.tsx` (404), and `route.ts` (API endpoints). The App Router is built on React Server Components by default, meaning components run on the server and ship zero JS to the client unless explicitly marked `"use client"`. This produces dramatically smaller bundles and faster Time-to-Interactive than the Pages Router. The cost is a steeper mental model — RSC vs Client Components, streaming, server actions — but the performance and SEO benefits make it the right default for new Next.js apps in 2026.