What is node.js excatly?

i am new to web sockets and node .. I am confused with different terminologies used with node.js. Few places it's mentioned it's a tool, few others it's a JavaScript program / any platform or some times it's an sdk at server side. Please tell me what exactly is this ode.js?

How do i write program using websockets.io in windows machine (server-side) for a client app written using web sockets. Or is there any better implementation than websockets.io for producing a faster response.. or high speed?

http://nodejs.org/ is the best source of info on the topic. This is the synopsis.

first of all you need to install "exress" module to use socket.io [for simplicity you should use "express". otherwise you can implement socket.io by other way too.]

To install express write in your terminal 

npm install express

Then install socket.io module for that you should write in teminal

npm install socket.io

Make file app.js Write code as below

 var express = require('express')
  , stylus = require('stylus')
  , nib = require('nib')
  , sio = require('socket.io');

app.listen(3001);
var io = sio.listen(app);

app.get('/', function (req, res) {
   res.redirect('/views/index.html');
});

io.sockets.on('connection', function (socket) {
      socket.emit('hi',{"This is first Message from socket.io"});
});

Then make file index.html in views folder

in it write 

<script type="text/javascript" src="http://code.jquery.com/jquery-1.6.1.min.js" > </script>
<script type="text/javascript" src="/socket.io/socket.io.js" ></script>

    <script>
         $(function(){
          var socket = io.connect();
              socket.on('hi',function(Message){
                   alert(Message); // Will alert "This is first Message from socket.io"
              });
         });
    </script>