OAuth 2.0: Authorization framework. Lets users grant third-party apps access without sharing passwords. "Login with Google" uses OAuth.
JWT (JSON Web Token): Self-contained token with claims. Server signs it. Client sends it with requests. Server validates signature without database lookup.
JWT structure: header.payload.signature (base64 encoded). Payload contains user ID, expiration, roles. Signature prevents tampering. Keep tokens short-lived ( min to hour).