In non-residential or commercial applications a cornice is the top molding or crown piece on a building or structure. It is often used in Greek architecture, and it is the top most piece in an Entablature. It generally runs on all four sides of the building.
RESIDENTIAL ROOF CORNICE
On outdoor residential applications a roof cornice is a molding that attaches the outside walls to the eaves. Again this serves as a decoration.
Indoors a cornice is a horizontal ornament molding that attaches the top of a room wall to the ceiling.