Blue layer, resin: functions as the a durable waterproof glue to keep the skimboard "sandwich" together.
Yellow layers, fiber: A light-weight sheet that bonds with resin. The main influence in flex resistance, crack resistance and overall durability. Largely responsible for board rigidity.
Gray layer, foam: The light-weight and stiff core of the board, providing buoyancy.
Graphic from Ajztek on Instructables
There are several variants of fibers and resins used in skimboards. In the rigidity department, there is not a preference debate. A stiff, flex resistant, skimboard will function the best in aiding holding speed when sliding over water and turning response. Boards flexing during sliding will be subject to addition drag from the water. There is also a minority of skimboarders who prefer flex to load weight into a poppy manuevers.
E-Glass Commonly referred to as fiberglass, this fiber is used in most skimboards, beginner to intermediate, to provide a cost efficient entry into wave riding. Some pros even ride these boards for the additional flex and pop they provide.
S-Glass A less common fiber which is 30% stronger and 15% stiffer than E-Glass. Often found in intermediate to advanced level boards.
Carbon Industry standard for pro level board construction. Carbon fiber is the lightest weight, most rigid, and most durable board fiber. While costing a premium, carbon will provide no performance set backs and should be expected in any top level skimboard build. Double carbon is standard. Extra carbon layering functions to amplifies stiffness, but lack of rebound can always be a concern for breakage!
Polyester Often paired with entry and intermediate level boards as a cost efficient and effective material. Polyester is stiffer which makes it relatively less strong when flexed. Polyester boards can be repaired with either polyester or epoxy.
Epoxy Sometimes at a premium, this resin can be found paired with any board fiber to provide the best rigidity and durable among resin types. Epoxy boards maintain their strength through flex and rebound. Pro level boards will pair carbon fiber with epoxy resin. Do not repair with polyester resins.
Vinylester Often referred to as a middle ground between polyester and epoxy characteristics where a rider can experience board flex they may desire, without sacrificing that premium structural integrity.