What's the difference between WebSocket and Socket.io?

The Socket.io encapsulates the WebSocket and includes other ways of connecting, such as Ajax. The reason is that not all browsers support WebSocket, and through the Socket.io package, you don't have to worry about what connection is being used. You can use Socket.io in any browser to establish an asynchronous connection. Socket.io contains the server side and the client's library, if the browser uses the Socket.io JS, the server must also be applicable. If you are aware that what you need is websocket, you can use WebSocket directly.

What's the difference between WebSocket and Socket.io?