[unit-testing] What is Mocking?
There are plenty of answers on SO and good posts on the web about mocking. One place that you might want to start looking is the post by Martin Fowler Mocks Aren't Stubs where he discusses a lot of the ideas of mocking.
In one paragraph - Mocking is one particlar technique to allow testing of a unit of code with out being reliant upon dependencies. In general, what differentiates mocking from other methods is that mock objects used to replace code dependencies will allow expectations to be set - a mock object will know how it is meant to be called by your code and how to respond.
Your original question mentioned TypeMock, so I've left my answer to that below:
TypeMock is the name of a commercial mocking framework.
It offers all the features of the free mocking frameworks like RhinoMocks and Moq, plus some more powerful options.
Whether or not you need TypeMock is highly debatable - you can do most mocking you would ever want with free mocking libraries, and many argue that the abilities offered by TypeMock will often lead you away from well encapsulated design.
As another answer stated 'TypeMocking' is not actually a defined concept, but could be taken to mean the type of mocking that TypeMock offers, using the CLR profiler to intercept .Net calls at runtime, giving much greater ability to fake objects (not requirements such as needing interfaces or virtual methods).
What is Mocking? .
Mocking is generating pseudo-objects that simulate real objects behaviour for tests
I would think the use of the TypeMock isolator mocking framework would be TypeMocking.
It is a tool that generates mocks for use in unit tests, without the need to write your code with IoC in mind.
Mock is a method/object that simulates the behavior of a real method/object in controlled ways. Mock objects are used in unit testing.
Often a method under a test calls other external services or methods within it. These are called dependencies. Once mocked, the dependencies behave the way we defined them.
With the dependencies being controlled by mocks, we can easily test the behavior of the method that we coded. This is Unit testing.