A hollow vessel or pot is a form or shape with the wood completely hollowed out from the hole on top. A vessel should have a consistent wall thickness all the way throughout the piece and maybe left a little thicker at the bottom so it doesn't fall over. Turning a thin consistent wall thickness in your vessel shows skill and leaves the piece feeling lite. Turning a vessel or pot with a small hole on the top takes practice and skill because you can't see anything your doing. The smaller the hole the more difficult because you risk rubbing the opening with your tool and don't have much room to move around to make a proper cut. When turning a vessel or pot I try not to let the size of the piece determine the final shape. Although some wood is very expensive, I never settle for a shape that doesn't feel right. I often sacrifice or waste some wood to accomplish a shape I am happy with instead of having a "big" piece.