Truck Terminology

Seriously folks, please answer for me because I really don't know the terms I need. Let's take an open top single unit truck or van (not a pickup) with low sides, the kind of truck that is ideal for gardening work, something like a Mitsubishi Fuso. What do you called the load carrying part? Is it just the bed, or is there another term for it? Is it ok to call it the trailer even though it's not being towed, i.e., it's fixed to the chassis and cab. I know that it could have a tailgate, but what is it called when the side (gate?) drops too? What is the correct term for the floor? i,e., the part that you put your load on.