Is the only difference that skimboarders are closer to shore and drop the skimboard upon running towards the waves? Or are there other major differences.

skimboarders ride on VERY shallow water and use their own momentum to move. surfboarders ride on waves and use the waves power to push them.

they also use very different boards

goosey is right about that and yes they do use different boards and surfboards are usually a lot longer and have fins