What’s the difference between npm run dev and npm run start in Next.js?

TL;DR: In Next.js, next dev is used to run the app in development mode. On the other hand, next start is used to run the app in production mode, but requires next build to be run first to generate an optimized production build.

Development

When running the Next.js app in development, you’ll want to use next dev:

next dev starts the application in development mode with hot-code
reloading, error reporting, and more.

Production

When building the Next.js app for production, you’ll want to use next build:

next build creates an optimized production build of your
application. The output displays information about each route.

• Size – The number of assets downloaded when navigating to the page
  client-side. The size for each route only includes its dependencies.
• First Load JS – The number of assets downloaded when visiting the page
  from the server. The amount of JS shared by all is shown as a separate
  metric.

Followed by either next start, when you want to start the production server:

next start starts the application in production mode. The
application should be compiled with next build first.

Or next export, when exporting the app as static HTML:

next export allows you to export your app to static HTML, which can be
run standalone without the need of a Node.js server.

For more information refer to Next.js CLI docs.