OAuth 2.0 is an industry-standard authorization protocol that enables applications to obtain limited access to user resources without exposing credentials, forming the foundation for secure delegated access across the modern web. It separates authentication from authorization, allowing resource owners to grant third-party applications specific permissions through structured flows while maintaining control over their data. The protocol's flexibility supports diverse client types—from high-security server applications to browser-based SPAs and native mobile apps—while its extension ecosystem (PKCE, DPoP, PAR, mTLS) addresses evolving security threats and deployment scenarios. Understanding OAuth 2.0's flows, token types, and security parameters is essential for building secure APIs, implementing single sign-on, and protecting user data in distributed systems.