A callback is a function which is called after completion of a current running task. In Nodejs, all APIs are fully supports the callbacks asynchronous i.e. Non-Blocking Code features of NodeJs.

Due to Non-Blocking features NodeJs makes highly scalable and processed high number of requests without waiting for any function response .

It will more cleared by Blocking and Non-Blocking examples...

Blocking Code Example

Commands To Install Nodejs on Ubuntu

 sudo apt-get update
 sudo apt-get install nodejs
 sudo apt-get install npm

Lets suppose we have created a txt file named "message.txt" as following..

Data Intensive Real time Applications (DIRT)
Data Streaming Applications
JSON APIs based Applications
Single Page Applications

Now, Create a js file named test_one.js which has the following code:

var fs = require("fs");
var data = fs.readFileSync('message .txt');  //Called  synchronousaly
console.log(data.toString());
console.log("Here is program ended");

Now run the "test_one.js"

$ nodejs  test_one.js

The output is...

Data Intensive Real time Applications (DIRT)
Data Streaming Applications
JSON APIs based Applications
Single Page Applications
Here is program ended

This code is run line by line as showing in output.

Non-Blocking Code Example

Create a  "test_two.js" file having following code

var fs = require("fs");
fs.readFile('message.txt', function (err, data) {
    if (err) return console.error(err);
    console.log(data.toString());
});
console.log("Here is program ended");

Now run the "test_two.js" by command

$ nodejs  test_two.js and see the output

The output is...

Here is program ended
Data Intensive Real time Applications (DIRT)
Data Streaming Applications
JSON APIs based Applications
Single Page Applications

In this example, controller does not wait for complete file reading but it just jumped to print "Here is program ended" and at the same time program without blocking continues reading the file.