Ford calls a step-side truck bed a “Flareside,” while a “Styleside” is what they call a “standard” truck bed. The terms “stepside” and “fleetside” are used by Chevrolet and GMC, respectively.
Flareside truck beds have fenders on the outside of the bed, and a ribbed step is usually built into the side of the bed between the cab and the rear axle. This style was used on the first pick-up trucks. Since the standard bed came out, the Flareside has also been called the “sport model.”
The Flareside bed has straight walls on the inside, so long things can be laid out flat across the whole width of the bed. But the width of the bed as a whole is less than the Styleside.
On a standard, or Styleside, truck bed, the fenders are inside the bed. Unlike the Flareside, the outside sides of the bed follow the lines of the cab. On each side of the bed, the fenders make an arch.
This makes it harder to stack long items in the bed, but it also makes the bed wider all around. But some truck models have these arches built into the bed so that there are different levels for loading.
The front and sides of the bed’s interior are made with channels that allow a piece of plywood or sheet metal to be added to make a flat deck across the whole width of the bed.