Authorization controls what users can do after authentication through three main models: RBAC assigns permissions to roles, ABAC uses attributes and context for fine-grained control, and ACL attaches permissions to individual resources. Real applications like GitHub and Stripe often combine these models. OAuth2 enables delegated authorization without sharing credentials, while JWTs and bearer tokens carry user identity and permissions across systems. The key is choosing the right combination of models and mechanisms based on your application's complexity and security requirements