This HTML5 API establish a Socket connections between a server and a web browser. Means a web browser and a server can have a bidirectional, full duplex communication to each other. Or you can say that a persistent connection is created between a client and the server and both can send data at any time. Websocket is still not fully supported by most of browsers. Socket.io is one of library available which comes with a client and server implementation. You can use this library if you are looking to imeplement websocket in your web app.