Check it out. This is a motor:



The deck is the flat surface at the top of the block that mates to the head. The cylinder sleeves are what the pistons move up and down in. In this motor (B18C) there is a water jacket, which is the space you see between the sleeves and the rest of the block. A motor that looks like this on the inside is called "open deck."

This is another B-series motor, but this time with a block guard installed:



Note that the block guard fills up the water jacket to reinforce the sleeves. If a motor has no water jacket to start with, it's called "closed deck."