The main difference between cast pistons and forged pistons lies in their manufacturing process and material properties. Cast pistons are made by pouring molten metal into a mold, resulting in a lower cost but less durable product. On the other hand, forged pistons are created by compressing and shaping metal under high pressure, resulting in a stronger and more rigid piston. Forged pistons are commonly used in high-performance applications where strength and reliability are crucial.