The aim of Socket.IO is to make real-time apps possible in every browser and mobile device. It supports several transport protocols, choosing the best one for the specific browser.
If you were to implement your application with WebSockets, it would be limited to the modern browsers supporting that protocol. Because Socket.IO falls back on so many alternate protocols (WebSockets, Flash, XHR, and JSONP), it supports a wide range of web browsers, including some old crufty browsers.
As the application author, you don't have to worry about the specific protocol Socket.IO uses in a given browser. Instead, you can implement the business logic and the library takes care of the details for you.
Socket.IO requires that a client ...
