JUnit.org Resources for Test Driven Development

To someone who is new to unit testing, the idea of mock objects can be confusing to say the least. However in this tutorial, we will focus on the concept of mocking in general. What is a mock object? What is it used for? Why can't I mock object XYZ? Let's look into these questions and maybe clear a bit of the air on the use of mock objects.
The Concept of Mocking