In my application i need to connect two socket.io node applications.Using socket.io-client we can do like this.But i don\'t know how socket.io-client works and where to include
You need to create a socket.io client in your first app:
var io        = require('socket.io').listen(server); // this is the socket.io server
var clientio  = require('socket.io-client');         // this is the socket.io client
var client    = clientio.connect(...);               // connect to second app
io.sockets.on('connection',function(socket) {
  socket.on('eventFiredInClient',function(data) {
    client.emit('secondNodeAppln', data); // send it to your second app
  });
});
And in your second app, just listen for those events:
io.sockets.on('connection', function (socket) {
  socket.on('secondNodeAppln', function(data) {
    ...
  });
});
There's a bit of a race condition because the code above doesn't wait for a connect event on the client socket before passing events to it.
EDIT see this gist for a standalone demo. Save the three files to a directory, start the servers:
node serverserver &
node clientserver
And open http://localhost:3012 in your browser.