Flareside is the designation Ford uses to refer to a stepside truck bed, while Styleside is their designation for a standard truck bed. Chevrolet and GMC use the terms stepside and fleetside respectively.
Fenders on the outside of the bed characterize the Flareside truck bed, typically with a ribbed step made into the side of the bed between the cab and the rear axle. The first pickup trucks were equipped with this style. Since the introduction of the standard bed, the Flareside is also known as the “sport model”. The inside of the Flareside bed has straight walls that enable long items to lay flat across the entire width of the bed. However, the overall width of the bed is less than the Styleside.
The fenders on a standard, or Styleside, truck bed are inside of the bed itself. The exterior sides of the bed continue the lines of the cab, unlike the Flareside. The fenders create an arched area on each side of the bed. This results in a narrower area in the bed to stack long items but also provides a wider overall bed width. However, some truck models incorporate these arches into the design of the bed to create a multilevel loading area. The front and sides of the bed's interior have molded channels that provide for the addition of a piece of either plywood or sheet metal to create a flat deck across the full width of the bed.