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Both are devices intended to ignite upon impact and to cause a primary charge to fire. A 209 primer is actually a primer, with an integrated anvil, contained within a cup. It is a complete device, which is intended to be held in place such that the primer surface is struck by a firing pin - this will cause the primer to ignite, and shoot hot flame and gas through the hole in the far side of the cup, setting off the main charge. A percussion cap is essentially a simple cup with an impact-sensitive charge held in place at the base. It does not have an integrated anvil, and is designed to be placed on a nipple in order to function. When the cap is placed on a nipple and struck by a hammer, the charge is trapped between the top of the nipple (which acts as an anvil) and the hammer, causing the charge to ignite, with the flame and gas going through the hole in the nipple, through to ignite the main charge. Typical percussion caps are not much more than 1/8" across (although there are larger 'musket' caps), and they are open on one end. A 209 primer is approximately 1/4" across, and enclosed, with a flange or rim on one end.
