There are too many articles and blogs around the internet you can Google it and find the same, I would just like to share some of the links which would be easy for beginners. When you would like to manage lot of objects and its dependencies effectively across application you might need to think about it.
What is Dependency Injection?
If you want to book a Bus Ticket to go to Chennai from Bangalore you might know lot of ways to do it e.g., book the ticket by Redbus, booking through KSRTC website, directly going to the private bus agents and booking it etc.,
What if you don’t want to worry about all of these things and just tell a friend and give him money to book it. Wouldn’t be so simple? How simple would it be to do that without you knowing the underlying details of how your friend is going to do it? Would be awesome right?
You have learnt Dependency InjectionJ. Kidding huh?
Please refer to this link this will give you good insights on what is dependency Injection
Want to be more precise in understanding the same, please refer to http://www.martinfowler.com/articles/injection.html