Before we dig deeper into the concepts of the recommendation system, let's see two real-world examples of recommendation engines that we might be using on a daily basis. The examples are shown in the following screenshots. The first screenshot is from Amazon.com, where we can see a section called Customers who bought this also bought, and the second screenshot will be from YouTube.com, where we are seeing a section called Recommended:

As you can see in the screenshot which we have taken from http://www.amazon.com, it shows a list of books on Java. So if you search for keyword Core Java
on Amazon.com for buying books, you will get a list books on Core Java. If you select one of those core java books now and click on it, you will be directed to the page where you will get the full description about the book: its price, author, reviews, and so on. It is here, at the bottom of this section you will get a link as shown above where it is mentioned Customers...