Sprite is an empty object to group other Sprite inherited objects as in your case happy and sad Bitmaps.
Body is a representation of the physical body in box2d, it does not have any visual representation, only numbers as dimensions (width and height), position, rotation and different forces. Usually inside ENTER_FRAME event you take this values as position and rotation and apply them to the Sprite inherited object, so it would move exactly like the body in box2d simulated world.
Ground is an empty dummy box2d object. If you don't provide parameters to createBody it means the defaults are used, which basically means you don't care what values it has. The most common example of such body usage is for joints.
To create a joint you usually need two bodies, but what if you would to attach body through joint to some random position in the air, or dynamic position as mouse cursor, well you can do it by creating dummy physics object Here is an example: http://appcodingeasy.com/Gideros-Mobile/Dragging-Box2d-object-in-Gideros-Mobile
In case of your example, then this body is used for EdgeShape which is usually an arbitrary shape used for creating world boundaries or in this case simply ground, so the dynamic object won't fall outside the screen