Confused in the Hello World example, the Node.JS way

Here is the typical hello world example given in most books for Node.JS -

var http = require('http');
    http.createServer(function (req, res) {
    res.**writeHead**(200, {'Content-Type': 'text/plain'});
    res.**end**('Hello World\n');
    }).listen(8124, "127.0.0.1");

I can understand that via the "require" html functionality has been put into the var http. so .createserver function would be written in that module, to which as a parameter for callback we are passing an anonymous function. However where is "writehead" and "end" defined? and what is it exactly?