Desktop GUI Design/Prototype/Mockup/Wireframe

Question

I'm working on a project at the moment and I'm currently looking for some sources regarding creating GUI mockups. In my case, I'm creating basic diagrams with the intended layout of the design to ensure it will work correctly and consistently.

What is this called? I've seen various names for it, including:

1. GUI Prototyping
2. GUI Mockups
3. GUI Wireframes

Which applies to my situation and are there any good resources to reaffirm my decision to create these diagrams?