Next.js · App Router · Fresh / Intern · Comparison
What is the App Router in Next.js, and how does it differ from the Pages Router?
Short Interview Answer
App Router uses the `app/` directory with React Server Components by default; Pages Router uses `pages/` with client-centric page components.
Detailed Explanation
App Router introduces nested layouts, loading/error UI, parallel/intercepting routes, and first-class server components. Routing is filesystem-based under `app`. Data fetching moved toward async server components and server actions rather than getServerSideProps/getStaticProps. Migration can be incremental, but mental models for caching and boundaries differ substantially.