An articulated flatbed truck
A smaller rigid flatbed truck with hinged up sides
A flatbed truck (or flatbed lorry in British English) is a truck which can be either articulated or rigid. The back is a flat, level 'bed' with no sides or roof.
Smaller flatbed trucks are rigid. The back has low sides which can be hinges. The sides can be hinged up when goods are on the back and hinged down when unloading goods.
Images for kids
British 1972 Guy Big J4 flatbed
International Harvester flatbed truck
A modern flatbed with an unusual load, held down by webbing ratchet straps
Ford F-650 flatbed tow truck