Definitions for "Case Back"
The bottom of the watch case that covers the movement.
The plate on the back of the watch by which you can access the battery. Most case backs are stainless steel, but some are hard plastic. A screw-in case back is one that requires a special tool to "unscrew" the case back. This ensures a higher level of water resistance. You can tell if you have a screw-in case back by the "notches" (usually 5 or 6) around the circumference of the case back.
The cover on the back of a watch which protects the movement. A case back normally snaps back into place or in the case of a waterproof model will be of the screw down variety to ensure water cannot permeate the seal. Information regarding each watch can usually be found on the outside of the case back.