Q: What is Callback?
A: 
callback in Node.js is an asynchronous equivalent for a function

It is a special type of function passed as an argument to another function

Callbacks help us make asynchronous calls

If one of the instructions in a program is expected to perform a lengthy process,
the main thread of execution gets blocked. The subsequent instructions can be executed only 
after the current I/O is complete. This is where callbacks come in to the picture.

setTimeout(function () { 
   console.log('This prints after 1000 ms'); 
}, 1000);

console.log("Hello World");