Prototyping is an essential part of usability design lifecycle. Prototypes are set up to check out new interface/interaction ideas. Usually they can be mocked up: first on paper, then on the machine.
In many cases I used paper, low fidelity (Low-Fi), prototyping to create walkthrough or use case scenarios. However, sometimes a high fidelity (Hi-Fi) prototyping is required to test more complex interactivity. Hi-Fi prototyping can be implemented in Flash, Adobe Photoshop or video.
Choose links on top to see different methods for prototyping.