What is storyboard? How to use it? Why?

A storyboard is the most often used tool for getting a sense of how an idea will work before putting it down on tape. It consists of a series of images, much like the panels of a comic strip, that give you an idea of how to compose different scenes.
How to use
Hitchcock was well known for storyboarding every shot in his films. In fact, he was so meticulous about it that he considered that phase of the production--drawing the storyboards--to be the actual process of making the film. For him, shooting the film was just a necessary evil; the making of the storyboards was where most of the creative work took place.
But the question arises how can drawing a few storyboards have such an effect on the final outcome of a film or video? The main reason is time. Storyboarding is done before the video ever shows up on tape. Rather than spending time with a fully charged and powered up camera in the hand trying to decide what and how to shoot the video, the artist sit in a room and go over the script in detail. In essence, he pretends he is shooting the video and he draws each shot as he goes. Then reviewing these drawings in sequence to make sure everything will go together the way he think it will. If it doesn't, its best to know this beforehand. That way, he can make changes before he shoot, sometimes eliminating the need for re-shooting.
Why:
To save yourself more trouble. Visualizing your ideas before you shoot allows you, by yourself, on your own time, to get an idea of what will and what won't work. True, this won't solve everything, but it can go a long way toward giving your production a professional finish.