Socket.IO primarily uses the WebSocket protocol with polling as a fallback option, while providing the same interface. Although it can be used as simply a wrapper for WebSocket, it provides many more features, including broadcasting to multiple sockets, storing data associated with each client, and asynchronous I/O.
Socket.IO provides the ability to implement real-time analytics, binary streaming, instant messaging, and document collaboration. Notable users include Microsoft Office, Yammer, and Zendesk.
Socket.IO handles the connection transparently. It will automatically upgrade to WebSocket if possible. This requires the programmer to only have Socket.IO knowledge.
Socket.IO is not a WebSocket library with fallback options to other realtime protocols. It is a custom realtime transport protocol implementation on top of other realtime protocols. Its protocol negotiation parts cause a client supporting standard WebSocket to not be able to contact a Socket.IO server. And a Socket.IO implementing client cannot talk to a non-Socket.IO based WebSocket or Long Polling Comet server. Therefore, Socket.IO requires using the Socket.IO libraries on both client and server side.
As of version 2.0, Socket.IO makes use of µWebSockets as the underlying WebSocket library.