php - Explanation Dependency Injection

Can anyone help me explaining in 'easy words'

1. When should I use dependency injection.


2. Why should I use it? ( Should I use it in combination OR not with MVC?)


3. What does in it easy words ? How is it working


4. Is it like creating a factory for recycling your code/scripts?


5. How does it interact and how do I call it when I use a front-controler ( For example )

Those questions are already on stack, but I still have some problems understanding them.

So I am really looking for a simple explanation.

I currently set up my projects the following way ( Dummy )
http://pastebin.com/WJau2CyZ
And I have no clue how exact to implement DI.