Websocket is the technology and socket.io an implementation AFAIK
"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 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 being 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."