Understanding stubs, fakes and mocks.

I have just started to read Professional Test Driven Development with C#: Developing Real World Applications with TDD

I have a hard time understanding stubs, fakes and mocks. From what I understand so far, they are fake objects used for the purpose of unit testing your projects, and that a mock is a stub with conditional logic into it.

Another thing I think I have picked up is that mocks are somehow related with dependency injection, a concept which I only managed to understand yesterday.

What I do not get is why I would actually use them. I cannot seem to find any concrete examples online that explains them properly.

Can anyone please explain to me this concepts?