What is the difference between authentication and authorization in a backend system?

Authentication answers 'who is this user?' (login, credentials, OIDC, etc.). Authorization answers 'may this user perform this action on this resource?' (roles, permissions, resource ownership). A user can be authenticated but not authorized. JWTs and session cookies are transport and storage mechanisms, not a substitute for a clear policy layer on the server. Always validate on the server; never trust client claims alone for sensitive rules.

AuthN: sign-in, verify password or SSO
AuthZ:  if (user.can('orders:read', orderId)) { ... }

Start simple: try this concept in a tiny project before moving to advanced tools.