This is the first part of article on Dependency Injection. It will give you an idea about what Dependency Inversion Principle is. There are other two parts of the article which explain how to implement DIP.
A curry of Dependency Inversion Principle (DIP), Inversion of Control (IoC), Dependency Injection (DI) and IoC Container
An Absolute Beginner's Tutorial on Dependency Inversion Principle, Inversion of Control and Dependency Injection
Introducing the LinFu Framework, Part IV: Simple.IOC – The Five Minute Inversion of Control Container