I\'m doing chat system with nodejs. To create a node server I can either use net or http module. So what is the difference and benefit
http is built on top of net (which handles the TCP layer underlying HTTP).
I imagine that your "chat system" will have browser-based clients, since node.js isn't great for GUIs/desktop apps at the moment. If that's the case, you'll most likely want to use http.
A node.js chat server has already been made - node-chat. You could build yours on top of this, or take a look at its source code for some ideas.