What is the difference between Socket.IO and raw WebSockets?

Key differences: (1) Protocol — raw WebSocket sends plain strings/binary; Socket.IO adds its own message format with event names and acknowledgments; (2) Interoperability — a standard WebSocket client cannot connect to a Socket.IO server (and vice versa) without Socket.IO's library; (3) Features — Socket.IO adds rooms, namespaces, auto-reconnection, event-based API, broadcasting, acknowledgments, and long-polling fallback; (4) Performance — Socket.IO has higher overhead per message due to its protocol layer; raw WebSocket is leaner for high-frequency binary data; (5) Complexity — Socket.IO is simpler to use for common patterns; raw WebSocket requires implementing patterns manually; (6) Bundle size — Socket.IO client adds ~45KB to the browser bundle. Choose raw WebSocket for performance-critical applications with custom protocols; choose Socket.IO for rapid development of standard real-time features.