For using Socket IO, node js is necessary? if not, Can someone give an example code?
Yes, node.js is needed for the server side... From the Github readme:
Socket.IO is a Node.JS project that makes WebSockets and realtime possible in all browsers.
See the Github page or the website for more information and sample code.
Not necessarily - you need a websocket server, this can be nodejs or a java websocket server - http://jwebsocket.org/ or even a .NET websocket server - http://superwebsocket.codeplex.com/
If you want a nodejs websocket server/client example - http://goo.gl/b7ncb