Bending is a simple process in which a metal sheet is pressed against a die of a certain shape by a ram or punch. This whole get-up is typically contained with a larger machine called a press brake, some of which can generate up to 200 tons of force, which is almost as much as I can generate with my right arm. (Okay, maybe not.)
For a succinct demonstration of bending, check out this video from the Society of Manufacturing Engineers:
Bending is used to make things like metal boxes, electrical enclosures, certain kinds of ducts, and the base or "foot" of Apple's LED Display and iMacs.