I heard this term ‘Dependency Injection’ (DI) a couple of months back, and since then it was at back of my mind and I was planning to read about it. It sounded like some mysterious concept, but it turned out to be a simple one.

DI (or Inversion of Control) is nothing but transferring responsibility of initialization of instance variable to the calling class. This helps in loose coupling and testing of classes.

A very good post on the topic:

http://jamesshore.com/Blog/Dependency-Injection-Demystified.html

Additional Info:
http://stackoverflow.com/questions/130794/what-is-dependency-injection