What socket.io is
Socket.IO is a library that enables real-time, bidirectional and event-based communication between the browser and the server. It consists of a Node.js server and a javascript client library for the browser. (which can be also run from Node.js)
What socket.io is not
Socket.IO is not a WebSocket implementation. Although Socket.IO indeed uses WebSocket as a transport when possible, it adds some metadata to each packet: the packet type, the namespace and the ack id when a message acknowledgement is needed. That is why a WebSocket client will not be able to successfully connect to a Socket.IO server, and a Socket.IO client will not be able to connect to a WebSocket server either.