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 )
href="http://pastebin.com/WJau2CyZ"
rel="nofollow">http://pastebin.com/WJau2CyZ

And I have
no clue how exact to implement DI.