Corrugated board is formed by gluing one or more sheets of thick paper liner together with flute, a corrugated (wavy) paper that gives cardboard its strength. The namesake comes from this inner, corrugated later. If there are 2 or more paper liners, the flute is glued in between the layers.
The reason the flute is shaped the way it is due to the strength of arches, long known to be one of the strongest shapes to use. In addition to strength, the hollow space that each arch creates allows pockets of air to insulate whatever the corrugated board may be holding.
The following pictures are common types of corrugated board in order of stength: