"A pier is a raised structure, including bridge and building supports and walkways, over water, typically supported by widely spread piles or pillars." (Abstracted from Google Wiki)
The wall caps, pier cap or pillar cap mean the top stones sitting on the wall, or a pillar. At the bottom of the pier caps, there are usually 4 water drops (1cm x 1cm), so the rain will not drop down on the surface of the wall or pillar and make them dirty.
The stone caps, column cap stone pier, stone pier cap made by China Stones Inc. are always in first quality and could produce ancient ones as per customer's picture.