In the App Router, the folder structure inside app/ defines your routes — there's no router config to write. Each folder is a URL segment, and special files within it define what that segment renders.
app/
page.tsx → /
about/page.tsx → /about
blog/page.tsx → /blog
blog/[slug]/page.tsx → /blog/:slug (dynamic segment)
shop/(sale)/page.tsx → /shop (route group — parentheses don't add to URL)
Each folder can contain reserved filenames, each with a specific role:
page.tsx → the route's UI (REQUIRED to make a segment routable)
layout.tsx → shared wrapper that persists across child routes
loading.tsx → loading UI (auto-wraps the page in <Suspense>)
error.tsx → error boundary for the segment
not-found.tsx→ 404 UI
route.ts → an API endpoint (instead of a page)
// app/blog/[slug]/page.tsx → matches /blog/hello, /blog/anything
export default function Post({ params }: { params: { slug: string } }) {
return <h1>Post: {params.slug}</h1>; // params.slug = "hello"
}
Square brackets create a dynamic parameter passed to the page via params. Variations: [...slug] (catch-all), [[...slug]] (optional catch-all).
app/(marketing)/about/page.tsx → /about (the (marketing) folder is ignored in the URL)
Parentheses create route groups to organize files or apply different layouts without affecting the URL.
File-based routing makes the URL structure obvious from the file tree — no central route configuration to maintain and keep in sync.
Knowing the special filenames (page, layout, loading, error, route) and the bracket conventions ([param], [...catchAll], (group)) is the core vocabulary of building any App Router app.