We'll start out with the basics. See the boxes that contain the pictures? Those are called panels.
|CALVIN AND HOBBES by Bill Watterson|
Panels are, as Art Spiegelman calls them, the Ur-language of comics, the basic building blocks of the medium. They control the action. While "panels" are typically thought of as boxes, a panel can actually take on any shape, such as a television screen, seen here from Frank Miller's BATMAN: THE DARK KNIGHT RETURNS:
To index cards, seen here in Will Eisner's THE SPIRIT.
Whatever panel you choose, just make sure it suits that particular moment in your story! For example, here's Neal Adams, tilting the panels diagonally so it gives an increased length for the falling Beast:
And changing the panel size alone can change the amount of tension in any given scene, as proven here by Steve Ditko in one of the greatest and most important Spider-Man moments of all time:
You can view some more effects of different panel shapes in some installments of Comics Techniques and Tricks!