`error.tsx` is a route-segment error boundary UI for runtime errors and can show a fallback with a reset action. `not-found.tsx` is rendered when you call `notFound()` or a route can’t be resolved, to show a 404-like page for that segment.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "in-the-app-router,-what-are-`error.tsx`-and-`not"
function explain() {
// Start from the core idea:
// `error.tsx` is a route-segment error boundary UI for runtime errors and can show a fallbac
}