A storyboard is something we use to plan our story, scene by scene, for a presentation or a movie.
We create a storyboard by showing pictures which represent what will happen in that particular scene. The storyboard can also be combined with a script to create a good idea on what the story/clip/skit/video/cartoon/movie would look like.
A storyboard is useful when you have to plan a story/clip/skit/video/cartoon/movie scene by scene, so that other people will get a good idea on what the thing that you are doing is going to be about.